Mixed ion‐electron conducting Li3P for efficient cathode prelithiation of all‐solid‐state Li‐ion batteries

All‐solid‐state batteries (ASSBs) using sulfide electrolytes hold promise for next‐generation battery technology. Although using a pure Li metal anode is believed to maximize battery energy density, numerous recent studies have implicated that Li‐ion anodes (e.g., graphite and Si) are more realistic candidates due to their interfacial compatibility with sulfide electrolytes. However, those Li‐ion ASSBs suffer from an issue similar to liquid Li‐ion batteries, which is a loss of active Li inventory owing to interfacial side reactions between electrode components, resulting in reduced available capacities and shortened cycle life. Herein, for the first time, we explore the potential of Li3P for cathode prelithiation of Li‐ion ASSBs. We identify that the crystallized Li3P (c‐Li3P) has room‐temperature ionic and electronic conductivities of both over 10−4 S/cm. Such a mixed ion‐electron conducting feature ensures that the neat c‐Li3P affords a high Li+‐releasing capacity of 983 mAh/g in ASSBs during the first charging. Moreover, the electrochemical delithiation of c‐Li3P takes place below 2 V versus Li+/Li, while its lithiation dominates below 1 V versus Li+/Li. Once used as a cathode prelithiation regent for ASSBs, c‐Li3P only functions as a Li+ donor without lithiation activity and can adequately compensate for the Li loss with minimal dosage added. Besides mitigating first‐cycle Li loss, c‐Li3P prelithiation can also improve the battery cyclability by sustained release of low‐dosage Li+ ions in subsequent cycles, which have been embodied in several full ASSBs by coupling a LiCoO2 cathode with various types of anodes (including graphite, in foil, Sb, and Si anode). Our work provides a universal cathode prelithiation strategy for high‐efficiency Li‐ion ASSBs.

All-solid-state batteries (ASSBs) using nonflammable and nonvolatile inorganic solid electrolytes (SEs) have been predicted to revolutionize current liquid Li-ion batteries (LIBs) from a safety perspective. 1,2Among various fast ion conductors, Li-based sulfide SEs have evolved to have room-temperature ionic conductivities comparable to commercial liquid counterparts (>1 mS/cm), making ASSBs operable at ambient temperature. 3Moreover, sulfide SEs are poised to couple with high-capacity electrode materials due to their combined mechanical rigidity and ductility. 4Li metal ASSBs (LM-ASSBs) are particularly interesting because using the LM anode is believed to maximize battery energy densities.However, implementing LM-ASSBs is more challenging than envisioned initially.In addition to well-known issues such as interfacial incompatibility-induced dendritic growth and low critical current density, 5,6 recent studies have revealed that the low yield strength of the pure LM could lead to its extrusion through the sulfide SEs (called Li creeping) and direct short-circuiting (Figure 1A). 7Li creeping might occur even in a considerably dense (~92%) and thick (~1000 μm) SE pellet loaded at low stack pressures (<10 MPa). 8Therefore, there has been renewed interest in Li-ion anode alternatives for ASSBs.
So far, three kinds of Li-ion anode architectures have been developed in sulfide-based ASSBs, including metal foil, composite powder, and blade-cast anodes (Figure 1B).0][11] Composite powder anodes typically consist of active materials (e.g., graphite, Si, and Li 4 Ti 5 O 12 ), carbon additives, and SEs.3][14] Generally, assembled full ASSBs by coupling these Li-ion anodes (LI-ASSBs) could achieve better cycling performances, especially at high areal current densities and loadings, compared to LM-ASSBs.However, despite these advantages, LI-ASSBs also suffer from an issue similar to liquid LIBs, which is a loss of active Li inventory owing to the initial interphase formation and permanent parasitic interfacial reactions between electrode materials/carbon additives and SEs (Figure 1B), resulting in reduced available capacities (Figure 1C) and shortened cycle life. 15,16Therefore, prelithiation is still indispensable to compensate for the active Li loss in LI-ASSBs, just like in liquid LIBs. 17evertheless, the prelithiation of LI-ASSBs has rarely been considered and studied.
Prelithiation technologies can be divided into anode prelithiation and cathode prelithiation according to Li compensation implemented at the anode and cathode sides. 18,19Compared to anode prelithiation, cathode prelithiation has advantages in terms of operating convenience and precisely controlled prelithiation degree, which could be achieved by simply blending sacrificial Li + -rich additives into the battery cathode. 20,21However, at least three stringent requirements must be met for cathode prelithiation additives (CPAs): (1) far higher gravimetric/volumetric capacities compared to active cathode materials; (2) Li + release completed below the cut-off charge potential of existing cathodes and no Li + intake occurred the lowest cathode discharge potential; and (3) minimum disturbance of the electrode ion/electron transport owing to side reactions or gas release.3][24][25] It is important to note that most CPAs are electronic insulators.Their use relies on compositing highly electron-conducting carbon/metal nanophases with high fractions (typically over 30 wt%), 26,27 significantly lowering practically available specific capacities and possibly causing severe decomposition of sulfide SEs. 28esides, these CPAs also have poor ion conductivities (except for Li 3 N), which might not be a problem in liquid batteries because the penetration of liquid electrolytes into porous electrodes is mainly responsible for the percolation pathways of Li + ions.However, SEs cannot flow or wet the electrodes as much as their liquid counterparts, leading to difficulties in constructing effective triple-phase interfaces. 29,30Therefore, preferred CPAs applicable in ASSBs might be mixed ion-electron conductors (MIECs), just like active cathode materials (e.g., LiCoO 2 or its delithiation forms).Very recently, Park et al. 31 first reported that ionconducting Li 3 N as a sacrificial CPA enabled improved cyclability of sulfide-based LI-ASSBs.However, cracks and vacancies could form inside composite cathode layers as the decomposition product of Li 3 N is N 2 gas, which might destructure triple-phase interfaces and increase battery internal resistance.
Bearing the above concerns in mind, we explore the crystallized Li 3 P (c-Li 3 P), an MIEC with room-temperature ionic and electronic conductivities of both over 10 −4 S/cm, as an excellent CPA candidate in LI-ASSBs.The neat c-Li 3 P without any compositing treatment affords a high Li +releasing capacity of 983 mAh/g (~2 Li + per formula unit) during the first charging of ASSBs, more than seven times the theoretical specific capacity of the typical cathode LiCoO 2 (Figure 1E). 32Meanwhile, c-Li 3 P shows a stable delithiation potential of ~1.2 V versus Li + /Li, and the lithiation of its charged product (i.e., elemental P) occurs below 1 V versus Li + /Li.These characteristics indicate that c-Li 3 P is only a Li + donor without Li + intake occurring in working voltage windows of most cathode materials and can achieve adequate compensation of the active Li loss with minimal dosage added.Besides mitigating irreversible capacity loss in the first cycle, c-Li 3 P prelithiation can also improve the battery cyclability by sustaining the release of low-dosage Li + ions in the subsequent cycles, which is identified in several sulfide-based LI-ASSB cells by coupling a LiCoO 2 cathode with various types of anodes (including graphite blade-cast anodein foil anode, and Sb/Si composite powder anodes).

| Synthesis of c-Li 3 P
In a typical synthesis, biphenyl (Bp, ≥99.5%;Aladdin) was dissolved in dry tetrahydrofuran (THF, ≥99.9%;Aladdin), and an excess amount of polished Li metal sheets was added.After stirring overnight, 1 mol/L Li-Bp solution in THF was obtained.Then, a given amount of purified red P powder (Aladdin, 99.9%) was charged to the above solution and held for 10 h.After that, as-prepared Li 3 P (a-Li 3 P) powder was harvested by centrifugation, rinsing with THF, and vacuumdrying at room temperature for 15 min.Finally, the c-Li 3 P powder was obtained by annealing the a-Li 3 P in a vacuum at 600°C for 4 h.

| Material characterizations
X-ray diffraction (XRD) patterns were recorded using a Bruker D8 Advance diffractometer with Cu Kα radiation (λ = 1.5406Å).A polyimide tape was used in this test to isolate Li 3 P from the air and prevent possible side reactions.The microstructures were examined using a Zeiss Sigma300 scanning electron microscope (SEM) and an FEI Titan G2 60−300 transmission electron microscope (TEM).X-ray photoelectron spectroscopy (XPS) measurements were carried out on a Thermo Fisher Scientific spectrometer with a monochromatic Al Kα X-ray source.conductivities Ionic/electronic conductivities of Li 3 P were measured based on ac impedance spectroscopy, which was performed using an Autolab PGSTAT 302 N electrochemical workstation in a frequency range of 5 Hz to 7 MHz with an amplitude of 10 mV.The effective partial electronic and ionic conductivities were measured with ion-blocking (stainless steel, SS) and electron-blocking (Li 6 PS 5 Cl, SE) electrodes in symmetric cell configurations.
To determine the electronic conductivity of c-Li 3 P, an SS|c-Li 3 P|SS cell was assembled.To determine the ionic conductivity of c-Li 3 P, Li-In|SE|Li-In and Li-In|SE|c-Li 3 P|SE|Li-In cells were assembled.All cells were tested at room temperature.

| Electrochemical measurements
The composite cathode was prepared by mixing an active material (2 wt% LiNbO 3 -coated LCO, home-made), Li 6 PS 5 Cl (MTI), and vapor-grown carbon fiber (VGCF; Showa Denko) in a mass ratio of 68:30:2.The percent content of Li 3 P was controlled based on Li 3 P in the composite cathode (i.e., LCO + Li 6 PS 5 Cl + VGCF).For example, 100 mg of the composite cathode was mixed with 5 mg of Li 3 P, resulting in 5 wt% Li 3 P in the cathode.For the graphite anode, a slurry was prepared by mixing 98 wt% graphite (S360-L2-H; MTI) and 2 wt% polyvinylidene fluoride (PVDF; Kynar) in 1-methyl-2-pyrrolidone.For the Sb anode, the Sb (≤20 μm; Aladdin) powder was mixed with Li 6 PS 5 Cl in a mass ratio of 7:3.As for the Si anode, the Si (1-5 μm; Aladdin) powder was mixed with Li 6 PS 5 Cl and VGCF in a mass ratio of 5:4:1.
For the fabrication of full cells, Li 6 PS 5 Cl powder (100 mg) was pelletized under 240 MPa in a polyether ether ketone mold with a diameter of 10 mm.Then, certain amounts of cathode and anode materials were filled on both sides of the pellet and pressed at 370 MPa to obtain a three-layered pellet.For half-cells, a Li-In alloy foil was used and attached to the SE side at 120 MPa.All full and half-cells were tested under a stacking pressure of 60 MPa.
All the galvanostatic charge-discharge tests were conducted on the LAND CT2001A battery testing system with cut-off voltages of 2.5−4.2V (vs.Li + /Li) at a constant current of 0.1 C (1 C = 137 mA/g) at 25°C.Cyclic voltammetry (CV) curves were evaluated on a CHI660 electrochemical workstation between 2.5 and 4.2 V versus Li + /Li.Galvanostatic intermittent titration technique (GITT) studies were also performed by applying a current density of 0.1 C for 10 min, followed by a 2-h relaxation.All the specific capacities of cells with the LCO cathode were calculated based on the mass of LCO.

| RESULTS AND DISCUSSION
Traditionally, Li 3 P is synthesized by the reaction of red phosphorus (P) powder and Li metal melt above 200°C. 33However, the reaction is highly exothermic and unavoidably causes safety concerns.In this work, Li 3 P was generated at room temperature using a solution-based chemical lithiation method and then crystallized at a high temperature.The overall synthesis procedures are shown in Figure 2A.A Li-biphenyl solution in tetrahydrofuran (Li-Bp/THF; see Supporting Information: Figure S1) was first prepared based on our previous method. 34The Li-Bp/THF reagent has a reduction potential of ~0.4 V versus Li + /Li, which is lower than the onset lithiation potential of red P (~0.7 V vs. Li + /Li).Adding red P powder in a 1 mol/L Li-Bp/ THF solution could readily produce the a-Li 3 P through a chemical lithiation reaction, which was then annealed at 600°C to obtain the c-Li 3 P. XRD patterns of the a-Li 3 P and c-Li 3 P are shown in Figure 2B, and both can be indexed to the hexagonal Li 3 P with a space group of P63/mm (JCPDS No. 74-1160).Furthermore, the c-Li 3 P shows far narrower and stronger reflections, indicating that the annealing treatment significantly improves the crystallinity of Li 3 P.The c-Li 3 P was further characterized by SEM and TEM.The SEM image (Figure 2C) shows that it consists of irregularly shaped micrometersized particles without apparent facets.The TEM (Figure 2D) and high-resolution TEM (Figure 2E) images show its polycrystalline structure with crystal domain sizes of a few nanometers.The lattice fringes of 0.264, 0.209, and 0.213 nm correspond to the (102), (103), and (110) crystal planes of hexagonal Li 3 P, respectively.The above results indicate the generation of polycrystalline Li 3 P.
The ac impedance analyses were conducted to determine the effective room-temperature conductivities of Li 3 P. First, using SS rods as ion-blocking electrodes, an SS|c-Li 3 P|SS cell was assembled, and the resultant impedance spectrum is shown in Figure 2F.In the lowfrequency region, a flat semicircle fully intersects the Z′ axis (real axis) without a visible Warburg tail.This result indicates that electronic conduction dominates the physical process rather than double-layer capacitance; however, the emergence of a semicircle instead of a single point further demonstrates low conductivity relative to metal materials. 35Therefore, the intercept of the semicircle at low frequencies corresponds to the electronic resistance (R e = 175.5 Ω).With a thickness of 0.99 mm and a calculated area of 0.785 cm 2 for the tested pellet, the electronic conductivity (σ e ) of c-Li 3 P is calculated to be 7.2 × 10 −4 S/cm.Subsequently, an electron-blocking configuration is needed to assess the ionic conductivity of c-Li 3 P. Here, Li 6 PS 5 Cl SE layers were used as ion-conducting and electron-blocking electrodes, while Li-In alloy foils were also used as reversible electrodes and Li reservoirs to avoid low-frequency polarization.Figure 2G shows the impedance spectrum of the five-layer Li-In|SE|c-Li 3 P|SE|Li-In symmetric cell, in which the intercept indicates the total ionic resistance (R total = 323 Ω), composed of one c-Li 3 P layer (R i ), two SE layers (2R SE ), and two SE|Li-In interfaces (2R SE|Li-In ).To determine the impedance contributions of SE layers and SE|Li-In interfaces, a Li-In|SE|Li-In symmetric cell was further constructed, and its impedance spectrum is plotted in Figure 2H.Accordingly, the R SE and 2R SE|Li-In values are about 34 and 33 Ω, respectively.Therefore, the effective ionic conductivity (σ i ) of c-Li 3 P is estimated to be 5.7 × 10 −4 S/cm.The above results clearly indicate that the c-Li 3 P is an MIEC with relatively high ionic and electronic conductivities of both more than 10 −4 S/cm at room temperature.In addition, the a-Li 3 P with a low crystallinity demonstrates ultralow electronic/ionic conductivities, which cannot be measured (Supporting Information: Figure S2).
Subsequently, the Li + donor property of c-Li 3 P (or a-Li 3 P) was investigated in a half-cell configuration.In the cells, Li 6 PS 5 Cl was used as the SE separator due to its high ionic conductivity (>10 −3 S/cm) and intrinsic mechanical ductility, and a Li-In alloy was utilized as the counter electrode.The working electrodes consist of c-Li 3 P (or a-Li 3 P), Li 6 PS 5 Cl, and VGCF with a mass ratio of 68:30:2.The two assembled cells with c-Li 3 P and a-Li 3 P are referred to as Li-In||c-Li 3 P + Li 6 PS 5 Cl + VGCF and Li-In||a-Li 3 P + Li 6 PS 5 Cl + VGCF, respectively.As shown in Figure 3A, the cell with c-Li 3 P shows a flat delithiation potential of ~1.2 V versus Li + /Li, similar to liquid cells with nanostructured Li 3 P/carbon composites containing significant contents (usually over 30 wt%) of conductive carbon additives.With a cut-off charge potential of 4.2 V versus Li + /Li, the cell delivers the first delithiation areal capacity of 1.704 mAh/cm 2 , which can be translated into a gravimetric capacity of 983 mAh/g based on the mass of c-Li 3 P (Figure 3B).It is worth mentioning that this high Li + donor capacity is achieved by only using 2 wt% VGCF as the electron-conducting additive in the electrode, a typical dosage used in fabricating composite cathodes in sulfide electrolytebased ASSBs.It is well known that conductive carbon additives might induce the decomposition of sulfide SEs and contribute to the extra capacity. 36Although the VGCF additive with a relatively low specific surface area (compared to other nano carbons such as Super P) is believed to minimize the negative effect, the capacity contribution from the oxidative decomposition of Li 6 PS 5 Cl is still possible.To this end, a cell with a cathode only containing Li 6 PS 5 Cl and VGCF (30:2 by mass) was fabricated.As shown in the inset of Figure 3A, a negligibly small capacity (<0.002 mAh/cm 2 ) is observed.This comparison unambiguously indicates that the electrochemical decomposition of c-Li 3 P dominantly contributes to the capacity of the Li-In||c-Li 3 P + Li 6 PS 5 Cl + VGCF cell, which does not rely on adding a large dosage of conductive carbon additives.Therefore, a specific capacity of 983 mAh/g for c-Li 3 P during the first charging is reasonable, corresponding to ~2 Li + per formula unit.As an MIEC, c-Li 3 P enables smooth Li + ion/electron transport within its interior, which is essential for an active material applicable in ASSBs, thus achieving high capacity utilization.Although the capacity value is 63.4% of the theoretical capacity for Li 3 P (1551 mAh/g), it is still much higher than the specific capacities of existing cathode materials (e.g., LiCoO 2 , 137 mAh/g), meeting the capacity criterion as a CPA.Such a high Li + donor capacity is also envisaged to enable a minimum use of c-Li 3 P as a CPA, thereby minimizing the specific energy loss of batteries.On the other hand, the a-Li 3 P electrode only provides a capacity of ~2 mAh/g under the same conditions due to its insulation for both Li + ions and electrons (Figure 3B).This result highlights that mixed ion-electron conduction is essential for a sacrificial CPA used in ASSBs.
To unravel the Li + donor mechanism of c-Li 3 P, the c-Li 3 P electrode, disassembled from the once charged Li-In||c-Li 3 P + Li 6 PS 5 Cl + VGCF cell, was analyzed by XPS.The P 2p XPS spectrum is shown in Figure 3C, which can be deconvoluted into two pairs of peaks.Except for the characteristic doublet peaks around 132.9 and 132.1 eV associated with the PS 4 3− of the Li 6 PS 5 Cl SE, 28 the intense doublet peaks located at 130.4 and 129.8 eV are also observed, which can readily be assigned to the P-P bond of elemental P. 37 Therefore, elemental P is identified as the charge product of c-Li 3 P, and the following reaction mechanism is verified: Li 3 P → 3Li + + P + 3e − .As is known, other typical CPAs (e.g., Li 3 N and Li 2 O 2 ) can entirely avoid the residue through gas products escaping from the electrodes, which has the advantage of minimizing battery energy loss. 23,38However, the gas release could also destructure the percolation network of electrodes and disrupt charge carrier transport, resulting in enhanced internal resistance.Given that the decomposition product is solid P, the c-Li 3 P as a CPA is anticipated to be superior regarding this.
To this end, we checked the c-Li 3 P + Li 6 PS 5 Cl + VGCF electrode upon first charging.Compared to the pristine electrode, the charged electrode remains dense without visible cracks or voids (Figure 3D).This observation shows that the c-Li 3 P as a sacrificial CPA is nondestructive to the solid electrode architecture.To evaluate the effect of the Li 3 P added on battery kinetics, one half-cell was prepared by using a LiNbO 3 -coated LCO cathode and a Li-In alloy foil as a reference/ counter electrode (denoted as Li-In||LCO).Another half-cell was prepared for comparison, with 5 wt% c-Li 3 P added to the cathode (denoted as Li-In||LCO + 5% c-Li 3 P).The two cells were pre-cycled two times by slow-scan CV (Supporting Information: Figure S3) and then subjected to GITT tests by applying a pulse current of 0.1 C (1 C = 137 mA/g) and a relaxation period of 2 h for each step.The obtained GITT profiles of the two cells are compared in Figure 3E, and the corresponding voltage polarization values (ΔV) for each current pulse are plotted in Figure 3F.Although higher specific capacity (based on the mass of LCO) is delivered by the Li-In||LCO + 5%c-Li 3 P cell due to the Li-compensation effect of c-Li 3 P, the polarization plots for two cells almost overlap, confirming the negligible negative influence of the Li 3 P CPA on the battery kinetics property.Further, the Li-In||c-Li 3 P + Li 6 PS 5 Cl + VGCF cell was cycled within a typical anode window of 0.01−2 V versus Li + /Li, and the first three charge-discharge curves are shown in Figure 3G.Clearly, the delithiation of c-Li 3 P mainly occurs above ~1.2V versus Li + /Li, but its lithiation potential is below ~1 V versus Li + /Li.As is known, existing cathode materials usually work in potentials higher than 2 V versus Li + /Li, which indicates that c-Li 3 P as a CPA can contribute active Li during battery charging, but almost no Li + intake occurs during battery discharging, meeting the voltage criterion as a CPA.Besides, the Li-In||c-Li 3 P + Li 6 PS 5 Cl + VGCF cell was also cycled within a cathode window of 2.5−4.2V versus Li + /Li. Figure 3H shows the selected galvanostatic charge-discharge curves for the 2nd, 10th, 50th, 100th, and 200th cycles.Surprisingly, despite almost no lithiation capacities (Figure 3H, inset), a considerable delithiation capacity of ~5 mAh/g is observed during the second cycle; then, the capacities gradually attenuate during subsequent cycles.Cumulative capacities as a function of the cycle number are depicted in Figure 3I, demonstrating that an extra Li + donor capacity of 89 mAh/g from the 2nd to 200th cycle can be provided by c-Li 3 P, except for the first capacity of 983 mAh/g.The incomplete capacity utilization (63.4%) during the first cycle could be related to the generation of the delithiated product (i.e., elemental P) on the surface of c-Li 3 P particles, forming a c-Li 3 P@P core-shell structure (Figure 3I, inset).As a result, the ion/electron-insulating P shells act as a barrier of charge carrier transport and thus block the Li extraction from residual c-Li 3 P cores during first charging.Despite this, the c-Li 3 P cores can still release the low-dosage Li + ions during subsequent cycles.Such a sustained release effect has never been observed before in other CPAs.Based on these intriguing characteristics of c-Li 3 P, it can be anticipated that (1) during the first battery cycling, c-Li 3 P as a CPA can compensate for a significant loss of active Li due to the interphase formation and (2) during subsequent cycling, it can offset the slight losses of active Li due to persistent electrode-electrolyte side reactions.Consequently, batteries' practically available capacity and cyclability can be improved simultaneously.
To evaluate the Li-compensation effect of c-Li 3 P, electrochemical characterizations of graphite||LCO full cells with various c-Li 3 P contents were carried out.For all full cells, the cathode areal capacity was controlled to ~1 mAh/cm 2 based on the theoretical capacity of LCO (137 mAh/g), and the anode areal capacity was set to ~1.3 mAh/cm 2 based on the first specific (lithiation) capacity of graphite obtained from a half-cell (Supporting Information: Figure S4), resulting in a negative/positive capacity ratio (N/P ratio) of ~1.3.Note that the anode is a blade-cast electrode composed of 98 wt% graphite and 2 wt% PVDF binder without any SE added.As is known, the highly electron-conducting graphite used in the routine composite powder electrode architecture would induce intense reductive decomposition of sulfide SEs, resulting in rapid-growing interfacial resistance and short battery life. 39,40Indeed, there has been no report as yet on the achievement of long-term cycling of full sulfide ASSBs with graphite/sulfide SE composite anodes.Here, the electrolyte-free electrode design aims to reduce the interfacial contact area between graphite and SEs to a two-dimensional plane, thereby minimizing those undesired interfacial site reactions.Even so, the pristine graphite||LCO full cell without the Li 3 P CPA delivers the initial charge/discharge capacity of 137/ 100 mAh/g at 0.1 C (Figure 4A), meaning ~27.7% loss of active Li during first-cycle charging.It is worth mentioning that the initial charge capacity of the cell (137 mAh/g based on the mass of the LCO cathode) is equal to the theoretical gravimetric capacity of LCO.This result suggests that the LiNbO 3 coating onto LCO can sufficiently suppress the interfacial oxidative decomposition of the Li 6 PS 5 Cl SE on the cathode side, and the firstcycle loss of active Li inventory in the cell can be attributed primarily to the interphase formation on the anode side during charging.With the c-Li 3 P CPA used, a voltage slope below the charge plateau of LCO is observed, which becomes more pronounced as the c-Li 3 P dosage increases.The additional signals could be related to the electrochemical decomposition of c-Li 3 P during charging.As a result, the charge plateau of LCO seems to shift entirely toward the high-capacity direction as the c-Li 3 P dosage increases.Also, the discharge plateau of LCO is substantially prolonged even though its voltage location almost remains invariant.In particular, the full cell with 5 wt% c-Li 3 P can deliver a discharge capacity of 133 mAh/g (based on the mass of LCO), approaching the theoretical value.This result confirms that the released capacity from only 5 wt% Li 3 P can fully compensate for the first-cycle capacity loss of a graphite|| LCO full cell.Besides, the full cell with 8 wt% c-Li 3 P underwent direct internal short-circuiting (Supporting Information: Figure S5), which could be attributed to the Li plating and dendrite growth on the anode side when an excess CPA was used.
The influence of the c-Li 3 P additive on cycling and rate performances of graphite||LCO full cells was also investigated.As shown in Figure 4B, the pristine full cell without c-Li 3 P shows poor cyclic stability with a capacity retention of 56.3% after 100 cycles at 0.1 C, which is substantially enhanced to 80.6% after adding 5 wt% c-Li 3 P additive.When the cycle number is extended to 250 times (running time of over 146 days), the cell with 5 wt% c-Li 3 P can maintain a reversible capacity of 79 mAh/g.This significantly enhanced cyclability could be ascribed to the sustained release of Li + ions from the residual c-Li 3 P that incompletely decomposes during the first cycle, as mentioned above.2][43][44][45][46] Simultaneously, we monitored the impedance changes for graphite||LCO and graphite||LCO + 5%c-Li 3 P cells during 100 cycles.As shown in Figure 4D, the two cells show a similar growth trend regarding overall cell resistance, suggesting a negligibly negative effect of the c-Li 3 P additive on cell kinetics.The rate capability of the two cells is also compared in Figure 4E,F.Although the graphite||LCO + 5%c-Li 3 P cell delivers a higher capacity than the graphite||LCO cell at each C-rate from 0.1 to 1 C, the capacity retentions of the former (81.0% at 0.2 C, 69.4% at 0.5 C, and 55.1% at 1 C) are almost identical to those of the latter (83.3% at 0.2 C, 72.4% at 0.5 C, and 59.8% at 1 C).This result confirms that using the c-Li 3 P additive is conducive to improving the available battery capacity but shows little adverse influence on the battery kinetics.Besides, the SEM images of the pristine and cycled LCO + 5%c-Li 3 P electrodes are shown in Supporting Information: Figure S6.Compared to the pristine electrode, the cycled one remains dense without visible cracks or voids after 100 cycles at 0.1 C.This observation demonstrates that minimum mechanical degradation occurs when the c-Li 3 P CPA is added to the solid cathode.
To demonstrate the general applicability of the c-Li 3 P CPA in LI-ASSBs, we assembled In||LCO, Sb|| LCO, and Si||LCO full cells.Note that, for In||LCO cells, a pure In foil (~100 μm, in large excess) was directly used as the anode; for Sb||LCO cells, the Sb composite anode composed of 70 wt% Sb powder and 30 wt% Li 6 PS 5 Cl was used; and for Si||LCO cells, the Si composite anode composed of 50 wt% Si powder, 40 wt % Li 6 PS 5 Cl, and 10 wt% VGCF was used.In all cases, the LCO cathode areal capacities were controlled at ~1 mAh/cm 2 .The areal capacities for Sb or Si anodes were set to ~1.3 mAh/cm 2 based on the corresponding

| CONCLUSION
In brief, we propose a solution-based chemical lithiation method to synthesize c-Li 3 P. Based on the ac impedance tests of assembled ion or electronblocking cells, we identify that the c-Li 3 P possesses room-temperature ionic and electronic conductivities of both over 10 −4 S/cm.The mixed ion-electron conducting c-Li 3 P demonstrates obvious advantages when used as a CPA in sulfide LI-ASSBs: (1) the neat c-Li 3 P affords a high Li + -releasing capacity of 983 mAh/g during the first charging, enabling a minimal dosage to compensate for the first-cycle capacity loss in full LI-ASSBs; (2) its electrochemical delithiation and lithiation mainly occur below 2 V versus Li + /Li and 1 V versus Li + /Li, respectively, meaning that it only functions as a Li + donor without lithiation activity; (3) its charged product is solid elemental P without gas by-product release, avoiding the destruction of solid electrode percolation networks; and (4) it can sustainably release low-dosage Li + ions in subsequent cycles, offsetting the persistent Li loss during battery cycling and enhancing the battery cyclability.These merits of c-Li 3 P have been demonstrated in several full ASSBs by coupling a LiCoO 2 cathode with various types of anodes.

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I G U R E 1 (A) Schematic of a Li metal anode used in a sulfide SE-based LM-ASSB.(B) Schematic of three kinds of Li-ion anodes used in sulfide SE-based LI-ASSBs.(C) Schematic of active Li loss that occurred in LI-ASSBs with representative anodes (In the foil anode, the Sb composite powder anode, and the graphite blade-cast anode).(D) Theoretical gravimetric/volumetric capacities of reported CPAs and their (de-)lithiation potentials.(E) Typical potential curves of c-Li 3 P and existing commercial cathodes (e.g., LiCoO 2 ).CPA, cathode prelithiation additive; LM-ASSB, Li metal all-solid-state batteries.

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I G U R E 2 (A) Schematic of preparation procedures of c-Li 3 P. (B) XRD patterns of a-Li 3 P and c-Li 3 P. (C) SEM image of c-Li 3 P. (D) TEM and (E) high-resolution TEM images of c-Li 3 P. (F) Electronic conductivity measurement of c-Li 3 P based on the impedance spectrum of an SS|c-Li 3 P|SS symmetric cell with SS ion-blocking electrodes.(G) Ionic conductivity measurement of c-Li 3 P based on the impedance spectrum of a Li-In|SE|c-Li 3 P|SE|Li-In symmetric cell with SE electron-blocking electrodes.(H) Impedance spectrum of a symmetric Li-In| SE | Li-In cell.SEM, scanning electron microscope; SS, stainless steel; TEM, transmission electron microscope; XRD, X-ray diffraction.

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I G U R E 3 (A) First charge voltage profiles of Li-In||Li 6 PS 5 Cl + VGCF, Li-In||a-Li 3 P + Li 6 PS 5 Cl + VGCF, and Li-In||c-Li 3 P + Li 6 PS 5 Cl + VGCF cells at 100 µA/cm 2 and (B) the corresponding voltage-gravimetric capacity profiles based on the mass of c-Li 3 P or a-Li 3 P. (C) XPS P 2p spectrum of the c-Li 3 P + Li 6 PS 5 Cl + VGCF electrode after first charging.(D) Top-view SEM images of the c-Li 3 P + Li 6 PS 5 Cl + VGCF electrode at its pristine and first charged states.(E) GITT profiles and (F) polarization plots of Li-In||LCO and Li-In||LCO + 5%c-Li 3 P cells during third discharging at 0.1 C. (G) First three charge-discharge curves of the Li-In||c-Li 3 P + Li 6 PS 5 Cl + VGCF cell in an anode window of 0.01 − 2 V versus Li + /Li.(H) Selected charge-discharge curves and (I) cumulative capacity during 200 cycles for Li-In||c-Li 3 P + Li 6 PS 5 Cl + VGCF cell in a cathode window of 2.5−4.2V versus Li + /Li.GITT, galvanostatic intermittent titration technique; SEM, scanning electron microscope; VGCF, vapor-grown carbon fiber; XPS, X-ray photoelectron spectroscopy.

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I G U R E 4 (A) First-cycle charge-discharge curves and (B) cycling performances for graphite||LCO full cells depending on the variation of c-Li 3 P contents at 0.1 C. (C) Long-term cycling performance at 0.5 C of graphite||LCO full cells with 5 wt% c-Li 3 P additive.(D) Impedance spectra for graphite||LCO and graphite||LCO + 5%c-Li 3 P cells during 100 cycles.Charge-discharge curves at various rates from 0.1 to 1 C of graphite||LCO full cells (E) without and (F) with 5 wt% c-Li 3 P additive.anode materials' first specific (lithiation) capacity in half-cells (Supporting Information: Figure S4), leading to an N/P ratio of ~1.3.As shown in Figure 5A−C, without the c-Li 3 P additive, In||LCO, Sb||LCO, and Si|| LCO cells deliver first discharge capacities of 96, 118, and 106 mAh/g at 0.1 C, respectively, representing Li losses of 32.9%, 15.1%, and 15.0% during first battery charging.After adding different contents of c-Li 3 P to the cathodes, In-, Sb-, and Si-based full cells show increased discharge capacities of 37, 10, and 8 mAh/g, respectively.More importantly, the cyclic stability of all cells is substantially enhanced (Figure 5D−F).