3D Nanocomposite Thin Film Cathodes for Micro‐Batteries with Enhanced High‐Rate Electrochemical Performance over Planar Films

High energy and high power density rechargeable micro‐batteries are a necessity for powering the next generation of flexible electronics, internet of things, and medical technology devices. In theory, significant improvements in the capacity, current and power densities of micro‐batteries would result if 3D architectures with enhanced interdigitated component interface areas and shortened ion diffusion path‐lengths were used. Further gains are achievable if the materials utilized have high crystalline quality and are preferentially oriented for fast lithium intercalation. In this work, this is achieved by creating epitaxial thin film cathodes comprised of nanopillars of LiMn2O4 (LMO) embedded in a supporting matrix of electronically conducting SrRuO3 (SRO). The first electrochemical study of such a 3D vertically aligned nanocomposite (VAN) that displays clear cathode redox signatures is provided, and demonstrates remarkable capacity retention under high‐rate regimes. The electrochemical performance is shown to be dependent on the nanopillar topography, namely the crystallographic orientation, nanopillar dimensions, and electrode/electrolyte interfacial surface area. This work offers a pathway to realizing 3D architectured micro‐batteries with high‐capacity retention under high‐rate conditions enabling fast charge capabilities.


Introduction
In order to achieve higher rate performance solid-state Li-ion batteries, fast lithium-ion dynamics are essential to avoid large DOI: 10.1002/aenm.202302053polarization effects at the electrodes.3] By nano-structuring battery electrodes such that diffusion path-lengths for both Li + and electrons are on nanometer scales, significant increases in the rate of lithium insertion/removal are achievable thus overcoming performance shortcomings due to polarization effects at the electrode interfaces. [1,2]f the known prominent cathodes, spinel LiMn 2 O 4 (LMO) is a promising micro-battery candidate for several reasons: 1) it has a 3D channel structure, which is desirable for efficient and highrate lithium diffusion 2) its high operational potential (3.7-4.3V vs Li/Li + ) enables high energy density 3) the fundamental properties and problems of LMO are known and well-studied, and finally 4) LMO is comprised of cheap, abundant raw materials and is environmentally benign.A diverse range of LMO nanostructures have been studied which includes nanoparticles, [4][5][6][7][8] nanorods, [9,10] nanowires, [11] nanotubes, [12] and porous spheres. [13]Nano-structuring of the cathode-electrolyte interface gives rise to high surfaces areas and high lithium-ion flux, hence enabling enhanced rate performances.However, extreme size reduction is often not favorable as it leads to more significant side reactions, impacting longevity and reducing discharge capacities. [1]On the other hand, if the nano-structures are too large, Li + dynamics hinder the total lithiation resulting in polarization. [5,14]Crystallographic orientation is also important, with the [001] and [110] orientations of LMO exhibiting higher capacities and rate performance as a result of preferential alignment of the Li + diffusion pathways for optimal ionic transport. [6,15][18] They have the potential to overcome the aforementioned diffusivity limitations and could be used in micro-battery technologies.VANs consist of two interdigitated epitaxial oxides, with a highly ordered vertically aligned pillar/matrix structure, grown on single crystal substrates. [19,20]Typically, their nanopillars have diameters of ≈20-100 nm and, due to their epitaxial nature, are of a single orientation which can be chosen based on the substrate orientation.[23][24][25] Also, VANs can be grown with high control of the interface orientation [26] and act as a scaffolds to stabilize challenging phases. [27]But crucially, by moving away from 2D planar architectures which remain constrained by low power and energy densities arising from their sluggish ion transport and limited areal surface interfaces, the interdigitated 3D architecture of VAN films could enable significant performance enhancements.
In this article, we report the systematic electrochemical study of a VAN cathode comprised of highly crystalline nanometerthick vertical pillars of oriented LMO embedded in a supporting matrix of highly electronically conducting (<10 −3 Ω cm) SrRuO 3 (SRO) (Figure 1a) grown on Nb-doped SrTiO 3 (Nb-STO), an electronic conducting substrate.][30][31][32] Since SRO is a perovskite with small lattice mismatch to Nb-STO (≈1%), it wets the substrate readily due to a low interfacial energy with Nb-STO, thus forming the matrix phase of the film. [19][35] We previously reported a Li x La 0.32 (Nb 0.7 Ti 0.3 )O 3 -(Ti 0.8 Nb 0.17 )O 2 (LL(Nb,Ti)O-(Ti,Nb)O) perovskite-anatase VAN grown on Nb-STO, which exhibited high Li + ionic conductivity (10 −4 S cm −1 at 25 °C) but poor electronic conductivity, attributed to the presence of an electronically insulating LL(Nb,Ti)O interfacial wetting layer between the substrate and electronically conducting (Ti,Nb)O nanopillars. [16]Further, a recently reported Li x La 0.5 TiO 3 -LMO (LLTO-LMO) perovskite-spinel VAN grown on SRO/Nb-STO was also shown to contain an interfacial LLTO wetting layer and did not exhibit clear LMO redox features in cyclic voltammetry experiments. [18]Such wetting layers are not uncommon in VAN films, [16,18,[36][37][38] but without careful consideration, could detrimentally impact their electrochemical performance due to being rectifying and very resistive.Specifically in the highlighted cases, the insulating LLTO-based wetting layer inhibits electronic transport between the electrode nanopillars and current collecting substrate, and may prevent the desired electrode redox from occurring. [32]Essentially, this is akin to not having the SRO current collector in the first place, without which the cathode redox does not occur, as documented in several early planar cathode (LMO and LiCoO 2 )/Nb-STO thin film studies. [28,30,32,39]In fact, a layer as thin as 1 nm of a material with a carefully chosen work function is all that is required to overcome, or amplify rectification at the Nb-STO substrate/cathode interface. [32]Thus, socalled work function engineering in the context of wetting layers is an essential consideration for battery VAN films to ensure successful operation.
The hypothesis of this work is that by using a LMO-SRO VAN grown on Nb-STO, any SRO wetting layer that forms underneath the LMO nanopillars would not be detrimental to the electrochemical performance of the VAN film.This is because there would always be a clear electronic pathway between the Nb-STO substrate to the LMO via SRO, and thus promote clear LMO redox activity when cycled.Indeed, we demonstrate that our LMO-SRO VAN cathode exhibits clear LMO redox behavior in galvanostatic cycling and cyclic voltammetry experiments.Further, we demonstrate that we can control both the orientation and pillar dimensions of our nanopillar LMO cathode, and identify that, analogous to cathode nanoparticles, there is an optimal pillar size for enhanced electrochemical performance for high C-rates.Our VAN films show remarkable capacity retention when operated at high rates (>85% up to 100 C), an ≈20% improvement over planar heterostructure thin films of SRO/LMO at comparable high current densities. [15]This is attributed to the nanometer path-lengths and 3D architecture within our VAN cathode.Further, our VANs exhibit extremely long-term cycle stability and excellent reversibility, with no appreciable capacity fade after 1000 cycles.This work offers a pathway to realize 3D architectured micro-batteries utilizing nanometer path-lengths, with selectively chosen crystallographic orientations that exhibit high capacity, rate performance, and long cycle lifetime.

Highly Crystalline LMO Nanopillars
Figure 1b shows a cross-sectional HAADF-STEM image of a VAN LMO-SRO film grown on Nb-STO (001).As we elucidate in later, both the LMO and SRO are (001) oriented.The dark LMO nanopillars are uniformly distributed within the bright SRO matrix.The nanopillars extend throughout the whole thickness of the film and are perpendicular to the Nb-STO substrate.STEM-EDX elemental maps (Figure 1c) show that there is clear phase separation between the two phases, further confirmed by the opposing undulations of Mn and Sr in the line scan.Atomic force microscopy (AFM) images mapping the topography of the film surfaces (Figure 1d) show clear pyramidal features, indicating that the LMO pillars terminate with the face centered cubic (fcc) close-packed (111) facet, the lowest surface energy facet for LMO. [40,41]This is also advantageous, since the (111) facet has the most dense manganese arrangement, which has been shown to form a stable cathode-electrolyte interphase (CEI). [6]The LMO nanopillars grow tilted with an angle of ≈55°with respect to the in-plane [100]/[010] Nb-STO direction (Figure 1b).This indicates that the LMO nanopillars grow with (111,−111), (−1-11), and (1-11) side facets to the SRO matrix, analogous to previous perovskite-spinel VANs grown on STO/Nb-STO. [18,33,42]s shown in previous VAN systems, the size and pitch of the nanopillar can be controlled by varying the growth temperature. [17,43]The average width of the nanopillar increases with increasing growth temperature, with values of 38(1), 47(2), and 70(3) nm determined from AFM for films grown at 500, 550, and 600 °C respectively (Figure 1e, data presented in Table S1, Supporting Information).Strictly, these values correspond to the maximum nanopillar width, taken at the surface of the film, owing to a subtly triangular cross-section shape.The thicknesses of films grown at different temperatures are 75 ± 5 nm (corresponding to a growth rate of ≈100 nm h −1 ), and so our pillars can be thought of as single-crystal nanopillars with width-toheight aspect ratios varying between ≈1:1 and 1:2.As would be expected, films grown at higher temperatures have larger, more defined pyramidal facets whereas the pillars grown at lower temperature are smaller and appear rounder in shape (Figure 1d). [19]he distribution of pillar sizes is roughly Gaussian (Figure 1e) with a greater standard deviation observed at higher temperatures.Thus, by controlling the processing parameters, we can control the morphology of the nanopillar array.
The orientation and lattice parameters of LMO-SRO VAN films grown on Nb-STO (001) between 450 and 650 °C were studied by high resolution XRD (Figure S1, Supporting Information).Varying the growth temperature results in subtle changes in XRD patterns of the (001) oriented films.Between 500 and 600 °C, clear LMO (004) reflections are observed with the highest intensity at 550 °C.From indexing, the LMO lattice parameters are determined as: (500 °C) a = 8.20(1) Å, (550 °C) a = 8.18(1) Å, (600 °C) a = 8.17(1) Å, well matched to bulk LMO (8.15-8.25 Å) [44] and other LMO films grown on STO. [15,28,29]The decrease in LMO lattice parameter with increasing growth temperature is ascribed to lower as-grown lithium content, consistent with increased Li volatility at elevated temperatures.Above 600 °C and below 450 °C, no distinct LMO reflections are observed (see Figure S1, Supporting Information for discussion).We also studied the orientation and lattice parameters of LMO-SRO VAN films grown on (001), (110), and (111) STO at 550 °C (Figure S2a, Supporting Information).All observed reflections in symmetric 2- scans that can be assigned to LMO show it has the same crystallographic orientation as the substrate, i.e., (111) LMO grows on (111) STO.XRD  scans (Figure S2b, Supporting Information) confirm that LMO grows epitaxially with the relationship [111]LMO//[111]STO.From indexing the 2- scans, the determined lattice parameters are: (001) a = 8.18(1) Å, (110) a = 8.23(1) Å, (111) a = 8.22(1) Å, again well matched to LMO films grown on STO. [15,28,29]The subtle differences in the lattice parameters for each orientation are due to variations in the as-grown lithium content.However, all are consistent with reported values for Li x Mn 2 O 4 between 0.5 < x < 1.0. [44]][47] We conducted HAXPES on a pristine LMO-SRO (001) sample grown at 550 °C (Figure S3, Supporting Information), revealing that manganese occupies the following oxidation states with the approximate percentages: Mn 4+ (48%), Mn 3+ (18%), and Mn 2+ (34%).The presence of Mn 2+ within spinel LMO (exclusively Mn 3+ /Mn 4+ ) has previously been attributed to a surface Mn 3 O 4 phase, where the Mn 2+ resides in the 8a tetrahedral Li site. [48,40,49,50]As no Mn x O y phases are detected in XRD (probing the average structure), we conclude that this Mn 2+ resides at the surface of the film/nanopillars, as reported in previous LMO films. [48]This may have implications for the electrochemical performance of these films (see Discussion).
For all 2- scans of LMO-SRO VAN films on Nb-STO [(001), (110), and (111)] between 500 and 600 °C, distinct out-of-plane SRO reflections are not resolved possibly because they are hidden by the STO (002) and LMO (004) reflections.To confirm the presence of SRO, we undertook a detailed high-resolution asymmetric X-ray reciprocal space map (RSM) of the (001) LMO-SRO VAN around the STO (113) reflection (Figure S2c, Supporting Information).This reveals a broad reflection that is indexed to LMO with a = 8.201(5) Å.The SRO (113) reflection overlaps the STO (113) indicating that the SRO is compressively strained (pseudocubic bulk lattice parameter a bulk = 3.93 Å [51] ) so that it is perfectly matched to the STO substrate, a SRO = a STO = 3.905 Å.To conserve cell volume, c SRO in the film would be expected to be >3.93Å.However, since the (00l) SRO peaks are not observed in 2- scans, this indicates that the LMO phase is controlling the lattice parameter of the SRO along the c-axis by vertical epitaxy.6]

Galvanostatic Cycling of LMO-SRO VANs
We now turn our attention to the electrochemical performance of our VAN films.The galvanostatic charge-discharge curves (Figure 2a) and dQ/dV profiles (Figure 2b) for all three orientations show the characteristic LMO redox process.The 1st curves (Figure S4b, Supporting Information) exhibit higher charging capacities with reversible capacities of 87, 58, 40 mAh g −1 , but with some irreversible processes occurring with capacities of ≈150, 50, and 45 mAh g −1 for the (001), (110), and (111) oriented films, respectively.All subsequent cycles show three distinct reversible electrochemical processes: two peaks between ≈4.0 and 4.2 V versus Li/Li + corresponding to the reversible two stage lithiation of LiMn 2 O 4 via Mn 3+ to Mn 4+ redox, [57] and a third peak at ≈4.35 V versus Li/Li + previously assigned to the Ru 4+ /Ru 5+ couple of LiMn 2-x Ru x O 4 . [58]Low level intermixing is observed in several VAN systems, [16,27] which is responsible for activating Ru redox in this system.
A specific capacity of >100 mAh g −1 over multiple cycles was seen for galvanostatic cycling of the LMO-SRO (001) VAN films (grown at 550 °C) at a 20 μA cm −2 current density (≈14 C) (Figure 2c).In fact, our (001) oriented VAN films achieve this capacity for 2000 cycles, gradually rising for the first ≈100 cycles before stabilizing to a steady capacity of 107 mAh g −1 retained for 2000 cycles (Figure S4, Supporting Information).Comparably, LMO-SRO (110) and (111) VAN films see specific capacities of 72 and 44 mAh g −1 for the 1000th cycle respectively.These gravimetric capacities correspond to areal and volumetric loadings of 1.42, 0.95, and 0.58 μAh cm −2 and 18.9, 12.7, and 7.7 μAh cm −2 μm −1 for (001), (110), and (111) oriented LMO-SRO VANs respectively.The trend in specific capacities (001)>( 110)>(111) was also observed in a previous study on epitaxial LMO thin films grown on Nb-STO. [15]There, the relationship between performance and LMO film morphology (crystallographic orientation, facet plane, and surface area) was elucidated, concluding that variations are due to differing ionic and electronic transport properties for each crystallographic orientation. [15]The fast Li + diffusion channels are preferentially aligned for the [001] and [110] directions, explaining why higher capacities are observed for these orientations. [6,7,15]These variations become more pronounced under high-rate conditions. [15]ur VAN cathodes exhibit a competitive gravimetric capacity compared with previously reported LMO thin films after 1000 cycles. [15]Remarkably, SEM images of (001) LMO-SRO VAN films show a retention of the VAN pillar features, even after 2000 cycles (Figure S4c, Supporting Information) indicating that the VAN structure is very stable upon cycling with no signs of fracture.Also, LMO reflections are still present in XRD 2- scans (Figure S4, Supporting Information), with no new reflections present indicating that no observable LMO-SRO impurity phases are formed upon cycling.This is further reinforced by dQ/dV plots, with the characteristic LMO redox retained and no additional peaks observed in the 1000th cycle (Figure 2b).
Further, we also investigated the impact of nanopillar size (Figure 1) on the galvanostatic cycling (Figure 2d-f).All our VAN cathodes show the typical redox features of LMO between ≈4.0 and 4.2 V, and the additional feature at ≈4.35 V assigned to ruthenium redox.Films grown at 500°C (38 nm), 550°C (47 nm), and 600°C (70 nm) exhibit specific discharge capacities of 38, 106, and 47 mAh g −1 for the 500th cycle respectively.An intermediate pillar size exhibits the highest capacity, which is the same as reported in many nanoparticle studies of cathode battery materials. [4,5,14,59]Again, the galvanostatic charge-discharge curves (Figure 2d) and dQ/dV profiles (Figure 2e) show the characteristic LMO lithiation process for all three films with differing pillar sizes.

Charge Storage and Kinetics
LMO-SRO VAN cathode films show remarkable rate capabilities and stability between 20 and 200 μA cm −2 (Figure 3).Our best rate-optimized film, a (001) oriented VAN cathode grown at 550 °C with an average LMO pillar diameter of 47 nm, retains 95% discharge capacity (71 mAh g −1 ) at 200 μA cm −2 (182 C).This is significantly better than previously reported planar LMO (110 nm thick) PLD thin films on Nb-STO [15] (Figure 3b, purple dashed lines).Interestingly, compared with VAN-like Li 2 MnO 3 -Au thin films on stainless steel, [60] which contain embedded Au nanopillars that are reported to enhance the electrochemical performance of the film with respect to comparable planar films, our optimized LMO-SRO VAN film achieves better capac-ity retention (95% versus 61%) and a higher discharge capacity (75 vs 60 mAh g −1 at 17/15 C respectively) under high-rate conditions (see comparisons in Table S2, Supporting Information).Concerning C-rates, all our VAN films show high retentions (>85%) when operated in high-rate regimes up to 100 C (Figure 3c, C-rates reported in Table S2, Supporting Information).Furthermore, all films show great stability and full capacity recovery when subjected to high and then slower current densities/C rates, exemplified by the stable discharge capacity and no appreciable loss when repeating the rate performance test (Cycle 61-120, Figure 3a).
The rate performance depends on both crystallographic orientation and pillar size, Figure 3 demonstrates the enhanced performance of (001) oriented films.In particular, our (001) VAN cathodes grown at 550 °C show higher rate discharge capacities in comparison with our other VAN films grown with different orientations or growth temperatures.When comparing films with different orientations grown at 550 °C and discharged at 200 μA cm −2 (benchmarked against 20 μA cm −2 ), (001) oriented VAN cathodes retain a remarkable 95% capacity after 120 cycles versus 43% (110) and 33% (111) (Figure 3b).This same trend in orientation dependence has been reported previously in epitaxial planar LMO films grown on Nb-STO. [15]However, our LMO-SRO VAN cathode films show significantly higher retention and gravimetric capacities at comparable current densities (dashed lines, Figure 3b).This improvement is exaggerated at high current densities, where all our VAN films, irrespective of orientation show enhanced retention compared with planar LMO films.
Concerning pillar dimensions, (001) oriented VAN cathode films grown at different temperatures, thus with different pillar sizes (Figure 1), exhibit differing rate performances (Figure 3).The highest capacity retentions and gravimetric capacities for all current densities are observed for films grown at 550 °C with an average LMO pillar diameter of 47 nm.Increasing the pillar size to 70 nm (film grown at 500 °C) retains a competitive capacity retention (74% at 200 μA cm −2 , ≈410 C), but results in a reduction of the gravimetric capacity (≈2x smaller vs 47 nm pillars).Nanosizing LMO pillars to 38 nm (film grown at 500 °C) also leads to lowered discharge capacities, a larger reduction than for VAN cathodes with larger LMO pillars.This is also accompanied by a reduced capacity retention versus current density (Figure 3b).However, the C rates for this film at comparable current densities are significantly larger (1510 C 200 μA cm −2 in Figure 3c, Table S3, Supporting Information).In fact, the capacity retention remains high provided the C-rate is < ≈100 C (79% at 40 μA cm −2 , 105 C).

Cyclic Voltammetry Studies
Cyclic voltammetry (CV) measurements were also conducted to further study the redox processes of the (001) VAN cathodes grown between 500 and 600 °C (Figure S5, Supporting Information).All films show the typical redox reactions of LMO at ≈4.0 V (LiMn 2 O 4 ↔ Li 0.5 Mn 2 O 4 ) and ≈4.15 V (Li 0.5 Mn 2 O 4 ↔ -MnO 2 ) when CVs are collected at sweep rates of 0.5-1.6 mV s −1 .Like the dQ/dV plots, an additional feature at ≈4.35 V is observed, tentatively ascribed to the Ru 4+ /Ru 5+ redox couple in Ru doped LMO. [58]All redox peaks are sharp and well-separated, indicating that the LMO nanopillars have good crystallinity, and that no side reactions are observed.Specific capacities from CVs (Figure S6, Supporting Information) are comparable with those observed for galvanostatic cycling (Figure 2).At 1.5 mV s −1 (corresponding to a charging time of 11 min), (001) LMO-SRO VANs grown at 550 °C (47 nm) achieve a capacity of 88 mAh g −1 versus 67 mAh g −1 (500 °C, 38 nm) and 64 mAh g −1 (600 °C).Again, intermediate pillar sizes exhibit the highest capacities.
We also examined the impact of pillar size on the charge storage behavior.To assess the nature of the charge storage process, our voltammetry data can be fit to the power law i(V) = a b where i(V) is the measured current and  is the sweep rate. [61]The parameters a and b (the b-value) are adjustable parameters that are determined from linear fitting plots of log i versus log .There are two well-defined conditions: a diffusion-limited contribution where i ∝  0.5 (b = 0.5); and a contribution that has been described as a capacitive contribution where i ∝  (b = 1.0), strictly indicating the current is not diffusion-limiting. [48]This capacitive contribution can include both double-layer contributions as well as pseudocapacitance (Faradaic) processes.When 0.5 < b < 1, this indicates that a combination of both diffusive and capacitive processes occur in conjunction.
Plots of log (peak current) versus log  (Figure 4a) indicate the power law i(V) = a b is obeyed for our VAN cathode films.The observed b-values for charging and discharging (Figure 4a) reveal interesting information about their behavior.All b-values are within the range 0.89-1.0(for charging times between 8 and 34 min) indicating that for both lithium insertion (discharge) and removal (charging) into our LMO-SRO VAN cathode, Li + diffusion is not rate limiting.
Further study of the CV data allows for the diffusive and capacitive contributions to be quantitively assessed.The current obeys the following equation: I = k 1  + k 2  0.5 where k 1 and k 2 are constants associated with the capacitive and diffusion-controlled intercalation processes, respectively.By plotting graphs of I −0.5 versus  0.5 , k 1 and k 2 can be determined.This analysis was performed on our films with the three different pillar sizes, with all three films obeying a linear relationship between I −0.5 and  0.5 (Figure S7, Supporting Information).We show three representative CVs collected at 1.5 mV s −1 with the capacitive current contribution shaded (Figure 4b).A clear increase in the diffusion-controlled processes is observed with increasing pillar size (500 °C (38 nm): 5%, 600 °C (70 nm): 19%).This data clearly demonstrates that the size and distribution of the nanopillars is integral to the performance of the LMO-SRO VAN cathode.
As discussed above, it should be stressed that the majority of the "capacitive" contribution is not as a result of electric doublelayer formation.Assuming a value of 10 μF cm −2 (a typical double layer capacitance [48] ), a perfectly flat 75 nm thick LMO-SRO VAN film with a surface area of 0.125 cm 2 (half the size of our substrates, accounting for ∼50:50 LMO:SRO volume ratio) would have a double layer capacitance of only ≈0.1 mAh g −1 .Even accounting for the surface topography of our films estimated from AFM (Table S5, Supporting Information), the doublelayer capacitance would be ≈0.15mAh g −1 , negligible compared with our observed gravimetric capacities (>60 mAh g −1 ).We also note that our VAN films exhibit a number of pseudocapacitive characteristics: 1) b-values equal to 1 for reasonably fast charge/discharge times; 2) small voltage overpotentials between redox peaks (Figure 2 and Figure 4).Additionally, we observe features that rule out the classic capacitive response: A) our CV data does not have a "box" shape, instead we observe clear redox features; B) we observe distinct plateaus in the galvanostatic chargedischarge curves (Figure 2).Thus, noting our observed b-values between 0.89 and 1.0, we can deduce the majority of the capacity in our VAN films is due to pseudocapacitive redox processes involving bulk Mn redox processes, not double-layer formation.
High levels of capacitive charge storage generally occur when diffusion rates are fast and diffusion path-lengths are small. [48]ence, we also assess the diffusion coefficients of different pillar sizes by utilizing the Randles-Sevcik equation [59] (Table S4, Supporting Information) using estimated areal areas of exposed nanopillars from our AFM statistics (Table S1, Supporting Information).Plots of i p versus  0.5 , where i p is the peak current, are linear confirming that the Randles-Sevcik equation is obeyed (Figure S8, Supporting Information).Diffusion coefficients of the order 10 −12 -10 −13 cm 2 s −1 are observed for both LMO redox peaks (Table S4, Supporting Information), within the typical range reported for LMO thin film cathodes determined from CV. [45,62,63] Again, the intermediate nanopillar width (47 nm, film grown at 550 °C) exhibits the highest diffusion coefficients, 2-3 x higher than the other VAN cathodes analyzed.

Discussion
This paper presents the first galvanostatic charge discharge cycling and cyclic voltammetry studies of vertically aligned nanocomposite cathode thin films.Our films are comprised of highly crystalline nanometer-thick vertical nanopillars of LiMn 2 O 4 cathode embedded in a supporting matrix of SrRuO 3 (Figure 1).We demonstrate that the nanopillar orientation, dimensions and distribution can be controlled by fine tuning the PLD process parameters (Figure 1); increasing the growth temperature results in larger nanopillars, that are distributed with a lower density.These factors impact the overall performance of the films, as we discuss below.
First, a relationship between the crystallographic orientation and electrochemical performance of our films has been demonstrated.VAN cathode films that are (001) oriented show the highest gravimetric capacity (Figure 2), and best rate performance (Figure 3), an observation reported previously on planar LMO thin films. [15]This is because the (001) orientation is well-aligned with the Li diffusion channels in LMO. [6,7]Our optimized (001) oriented cathode films show comparable gravimetric capacities compared with previously reported epitaxial planar LMO thin films grown on Nb-STO, [15] also providing a gravimetric capacity >100 mAh g −1 for 2000 cycles with negligible capacity fade (Figure S4a, Supporting Information).The VANs have (111) LMO faceting (Figure 1d), which has been shown to mitigate Mn dissolution into the electrolyte, thus improving cycle stability. [6]ext, we demonstrate that the topography of the VAN films has a significant effect on the gravimetric capacity, rate performance, diffusion coefficients, and nature of charge storage, with an optimum LMO pillar size observed.We found that the highest capacity and optimum rate capability and gravimetric capacity performance were achieved for nanopillars with an average diameter of 47 nm (films grown at 550 °C).Either increasing or decreasing the nanopillar width results in a reduction in capacity and electrochemical performance (Figure 2, Figure 3; Figure S6, Supporting Information), an observation that has also been reported for cathode nanoparticles. [4,5,14,64]For LMO nanoparticles, optimal performance is found for a diameter of ≈40 nm, [4,5] closely matched to our observations with nanopillars.Our VAN nanopillars are analogous to nanoparticles, but have a controlled crystallographic orientation, either (001)/(110) or (111), and are embedded in SRO (a current collecting matrix) so lithium insertion only occurs in one direction.
The trends in gravimetric capacity versus pillar size can be explained by two competing effects: the balance between pillar volume and the cathode/electrolyte surface area, which we deduce below from detailed analysis of our CV experiments (Figure 4) and AFM measurements.When nanosizing below ≈50 nm, the increased quantity of narrower nanopillars results in a greater total interfacial surface area (both pillar sidewalls with SRO and the top pyramidal termination facets) (Table S5, Supporting Information).Previous studies on cathode nanoparticles have demonstrated that there is a significant reduction in amount of lithium that can be extracted smaller nanoparticles (<≈40 nm diameter). [4,14]This phenomenon may result from LMO having a Mn 3 O 4 (Mn 2+ /Mn 3+ ) surface termination phase, supported by both experimental observation [40] and simulations. [49,50]Here, the Mn 2+ resides in the 8a tetrahedral lithium site, potentially blocking Li + transport and is redox inactive in the 4 V regime. [40]As the nanoparticle size is reduced, this redox inactive Mn 2+ layer contributes a greater portion of the overall nanoparticle volume, thus resulting in a greater volume of redox inactive material and a lower overall gravimetric capacity.This same argument has also been applied to thin films. [48]Our HAXPES data (Figure S3, Supporting Information) on a pristine LMO-SRO (001) VAN grown at 550 °C reveals 34% of the manganese present is in the redox inactive +2 oxidation state, a significant percentage.Hence, analogous to nanoparticles, as the pillar dimensions are reduced, the redox inactive Mn 2+ surface phase comprises a greater percentage of the overall LMO nanopillar volume.Consequently, this results in a reduction of the overall discharge capacity with decreasing pillar size (Figure 2f).
For LMO nanopillars > ≈50 nm in width, VAN cathodes have a lower density of wider LMO pillars.While there is an increase in the surface area (allowing higher Li flux), each pillar is now larger (Table S5, Supporting Information) and thus requires more lithium per pillar to fully lithiate.Now, the lithium dynamics become crucial to the charging mechanism.If ion transport is sluggish, full lithiation throughout the nanopillar cannot be achieved at high rates. [5]For our VANs, the diffusion coefficients are of the same order of magnitude (10 −12 -10 −13 cm [2] s −1 , Table S4, Supporting Information) and the b-values are comparable (0.89-0.99, Figure 4a), with b ≈1 indicating that the charge storage processes are not diffusion-limited.Hence, it can be concluded that Li diffusion is not responsible for the reduced gravimetric capacity of larger pillars, because the diffusional processes in the 550/600 °C grown VANs are not drastically different.Instead, we conclude the pillar dimensions are responsible.Increasing the LMO pillar dimensions from 47 nm (550 °C) to 70 nm (600 °C) increases the average pillar volume by ≈2.3x (Table S5, Supporting Information).Thus, the larger pillars require approximately twice the amount of lithium per pillar to fully lithiate, and consequently a 2x increase in surface area between the VAN film and electrolyte is required to maintain the same level of Li flux.However, noting that these LMO pillars are embedded such that only their pyramidal facets are exposed to electrolyte, there is only a ≈1.2x increase in surface area for the wider pillars (Table S5, Supporting Information).Hence, under fast charging regimes, the modest increase in surface area for the larger pillars is insufficient to supply the required lithium to fully charge, as emphasized by the surface area/volume ratio (Figure S6, Supporting Information).We also note that the lower density of pillars (Table S1, Supporting Information) will also impact the delivery of lithium in the electrolyte to the VAN cathode.
Consequently, the larger pillars are not fully lithiated, resulting in reduced capacity (Figure 2f).
The aforementioned morphology and topography effects have a more pronounced impact on the electrochemical performance when the VAN cathodes are operated at high current densities (Figure 3a).The achievable gravimetric capacity at high rates is dictated by the pillar dimensions and orientation, with the best performing films having (001) oriented pillars, and the right balance between a high-volume percentage of active material (larger pillars) that can fully lithiate quickly (smaller pillars).The 70 nm diameter nanopillars cannot fully lithiate fast enough at higher current densities, whereas the 38 nm pillars have a lower overall volume of high-capacity active material due to inactive Mn 2+ surface layers, both resulting in a fast drop off in gravimetric capacity with increasing current density.But, for the 47 nm (550 °C) pillars, these effects balance out resulting in a cathode exhibiting excellent rate performance, capacity retention, and high gravimetric capacity (Figure 3).Thus, our results demonstrate there is a sweet-spot in the optimum pillar dimension.In fact, all our LMO-SRO VAN films show high-capacity retention (>85%) up to 100 C, albeit with lower gravimetric capacities for non-optimal nanopillar widths and crystallographic orientations.

Conclusions
In summary, we have developed 3D architectured vertically aligned nanocomposite (VAN) thin film cathode with nanometer path-lengths comprised of interdigitated LMO and SRO.By growing (001) oriented films, well aligned to the Li diffusion channels, together with optimized pillar dimensions, our VAN cathode exhibits high discharge capacity (>100 mAh g −1 for 2000 cycles) and high-capacity retention when cycled at high current densities (>85% up to 100 C).The improved electrochemical performance over planar films under high-rate conditions is enabled by the 3D VAN architecture plus the careful balancing of the surface area and pillar size such that a large volume of high-capacity cathodic material is present with pillars that can fully lithiate under highrate conditions.Further, because our nanopillars are terminated with the (111) facet, which helps mitigate manganese dissolution into the electrolyte, our VAN cathode exhibits high stability and negligible capacity fade for over 1000 cycles.This accumulates into a high-rate 3D cathode thin film that could be implemented in next generation micro-batteries, either as a stand-alone cathode, or as a current-collecting layer that contributes capacity via inclusion of cathodic material.

Experimental Section
LMO-SRO Target Synthesis: A composite target was prepared by mixing LMO (Sigma-Aldrich, now Merck) and SRO (Toshima Manufacturing Co.) in a 1:1 ratio by volume, pelletizing, and sintering in air at 900 °C for 10 h.
Pulsed Laser Deposition Experiments: Films were grown by pulsed laser deposition (PLD) using a KrF excimer laser with a wavelength of 248 nm.Before growth, (001), (110), or (111) oriented SrTiO 3 /Nb-doped SrTiO 3 (0.5% wt.Nb, 5 × 5 × 0.5 mm) substrates were cleaned sequentially in acetone and propan-2-ol in an ultrasonic bath for 5 min.Prior to deposition, the PLD chamber was evacuated to a base pressure of at least 10 −6 mbar before filling to a growth pressure of 0.13 mbar O 2 .The target was preablated for 5 min with a fluence of 2.3 J cm −2 at 5 Hz.Films were grown on with the following growth conditions: T sub = 500-600 °C, F = 2.3 J cm −2 , pO 2 = 0.13 mbar,  = 2 Hz, substrate-target distance = 45 mm.
Thin Film X-Ray Diffraction Characterization: Films were characterized with high resolution X-ray diffraction (XRD), performed on a Panalytical Empyrean vertical diffractometer using a Cu K X-ray radiation source with a wavelength of 1.5418 Å.Three types of scans were conducted: 2-,  and reciprocal space maps (RSM).
Electron Microscopy Characterization: Scanning electron microscopy/energy-dispersive X-ray spectroscopy (SEM/EDX) data was obtained using a TESCAN MIRA3 FEG-SEM (FEG = field emission gun) microscope.SEM images and EDX maps were acquired at 5/15 kV and 30 kV acceleration voltages respectively.A FEI Helios Nanolab focused ion beam scanning electron microscope was used to prepare a lamella for transmission electron microscopy (TEM) studies.High-angle annular dark-field scanning transmission electron microscopy (HAADF STEM) images and energy-dispersive X-ray (EDX) maps were acquired using two transmission electron microscopes: a Thermo Scientific (FEI) Talos F200X G2 and a Thermo Scientific Spectra 300, operated in the scanning mode at 200 kV and 300 kV respectively.The transmission electron microscopy data were analyzed using the Thermo Scientific Velox software.
Atomic Force Microscopy: Tapping mode atomic force microscopy (AFM) was performed on selected films using a Bruker Multimode 8 AFM controlled by Nanoscope (Version 8.15).Monolithic silicon AFM tips with a reflective aluminium coating (Tap300Al-G) and a tip radius of 10 nm was used.
Electrochemical Characterization: For electrochemical characterization, films were transferred to an argon atmosphere glovebox (typically < 0.5 ppm H 2 O and O 2 ) and placed on a hot plate for 15 min at 125 °C to remove any water content.Then, electrochemical cells were assembled using swagelok-like EL-CELL PAT-Cells with a 2-electrode setup, containing PAT-cores (ECC1-00-0210-P/X) containing stainless steel plungers, the thin film to be studied grown on Nb-STO, a Whatman glass fiber separator, 60 μL of 1.0 m LiPF 6 in 1:1:1 ethylene carbonate, dimethyl carbonate, and diethyl carbonate (EC:DMC:DEC) and lithium metal anode.Electrochemical measurements were performed at 20 °C using an Arbin LBT20084 potentiostat and cycled galvanostatically between 3.5-4.5V with currents between 5 and 125 μA.Cyclic voltammetry measurements were conducted using a Biologic SP200 potentiostat with a 2-electrode PAT-Cell setup using the same cell conditions.All gravimetric capacities in this work were calculated by the following equation: Gravimetric Capacity ( mAh g −1 ) = Measured Capacity (mAh) LMO ( g cm −3 ) × film thickness (cm) × film area ( cm 2 ) × volume fraction of cathode (1)   where  LMO is the theoretical density of LMO (4.3 g cm −3 ), the film area is 0.25 cm 2 (the substrate size, 5 × 5 mm) and the volume fraction is 0.45 (corresponding to 45:55 ratio of LMO:SRO).The film thickness was determined from cross-sectional STEM/SEM grown on Nb-STO substrates.The volume fraction of LMO was determined to be 45% independently from both STEM phase analysis and STEM-EDX maps by comparing the Sr:Mn ratio over a large area.Reported areal (μAh cm −2 ) and volumetric (μAh cm −2 μm −1 ) capacities were determined from the total film dimensions (film thickness and area), inclusive of SRO.
X-Ray Photoelectron Spectroscopy Characterization: Hard X-ray photoelectron spectroscopy (HAXPES) was performed using the monochromated Ga K X-ray radiation (9250 eV, 3.0 mA emission at 210 W, microfocused to 50 μm) and an EW-4000 high-voltage electron energy analyzer (HAXPES-Lab, Scienta Omicron GmbH).The instrument has a base vacuum pressure of ≈5 × 10 −10 mbar.Survey and core level spectra were measured using 500 and 200 eV pass energies (using a 1.5 or 0.8 mm wide entrance slit), with a total energy resolution of 2.0 and 0.6 eV respectively, as measured using the FWHM of the Au 4f 7/2 core level on a clean gold reference sample.Binding energy scale calibration was performed using the O lattice photoelectron peak at 529.5 eV.Analysis and curve fitting was performed using Voigt-approximation peaks inbuilt within CasaXPS, following ref.[48].

Figure 1 .
Figure 1.Structural characterization of LMO-SRO VAN films.a) Schematic of our LMO-SRO VAN cathode b) Low magnification cross-sectional HAADF-STEM image of the LMO-SRO VAN grown on Nb-STO (001) showing clear LMO pillar features (dark) that are uniformly distributed throughout the SRO matrix (bright) c) Mn(purple)/Sr(green) STEM-EDX map with corresponding line scan (taken from pink line in (b)) confirming separation between the two phases d) Top-face AFM images of (001) LMO-SRO VAN films grown at 500 °C, 550 °C, and 600 °C showing clear pyramidal facets corresponding to the (111) termination of LMO.The pillar features increase in size with increasing growth temperature e) Maximum pillar widths determined from AFM images of the film surfaces showing clear increase in average width (AW) and standard deviation (SD) with increasing temperature.

Figure 2 .
Figure 2. Galvanostatic cycling performance of LMO-SRO VAN films.a) Charge-discharge curves and b) dQ/dV profiles for (001), (110), and (111) LMO-SRO VAN films grown at 550 °C showing the typical redox features of LMO.c) Cycle discharge performance for all three orientations showing high stability up to 1000 cycles.d) Charge-discharge curves e) dQ/dV profiles and f) cycle performance for (001) oriented LMO-SRO VAN cathodes grown between 500-600 °C with different nanopillar dimensions.Again, the typical redox features of LMO are observed.

Figure 4 .
Figure 4. Cyclic voltammetry data for LMO-SRO VAN films grown between 500 and 600 °C.a) Plots of log(Peak Current) vs. log(Scan Rate) with determined b-values for the 4 and 4.15 V redox peaks during charge and discharge.b) Capacitive contribution (shaded) for LMO-SRO VAN films at 1.5 mV s −1 .