A Cost‐Effective Production Route of Li4Ti5O12 Resisting Unsettled Market and Subsequent Application in the Li‐Ion Capacitor

Outstanding materials and novel device structures are key factors in satisfying the increasing demand for energy storage. Li‐ion capacitors, as one typical model of asymmetric supercapacitors, benefit from the battery's ultrahigh specific energy and supercapacitor's superlarge specific power to meet the balanced electrochemical energy storage requirements. The inherent and irreplaceable advantages make spinel lithium titanate an optimal candidate for power batteries and Li‐ion capacitors. However, upstream market volatility, including price spikes and supply shortages, tremendously threatens spinel lithium titanate's production, sale, and application. Lithium hydroxide hydrate is an alternative raw material to synthesize spinel lithium titanate synthesis, which can help address these concerns. The indirect production path generates high‐quality lithium carbonate as an intermediate product. Spinel lithium titanate is synthesized preferably via another economically and technically more efficient method that skips the isolation step after carboxylation. The electrochemical performance of the resulting spinel lithium titanate is evaluated to be better than that of commercial competitors. The spinel lithium titanate negative electrode‐based lithium‐ion capacitor achieves a specific energy of 89.5 Wh kg−1. These results demonstrate the success of efforts to find a feasible and affordable synthesis route to mitigate market risks.

Outstanding materials and novel device structures are key factors in satisfying the increasing demand for energy storage.Li-ion capacitors, as one typical model of asymmetric supercapacitors, benefit from the battery's ultrahigh specific energy and supercapacitor's superlarge specific power to meet the balanced electrochemical energy storage requirements.The inherent and irreplaceable advantages make spinel lithium titanate an optimal candidate for power batteries and Li-ion capacitors.However, upstream market volatility, including price spikes and supply shortages, tremendously threatens spinel lithium titanate's production, sale, and application.Lithium hydroxide hydrate is an alternative raw material to synthesize spinel lithium titanate synthesis, which can help address these concerns.The indirect production path generates high-quality lithium carbonate as an intermediate product.Spinel lithium titanate is synthesized preferably via another economically and technically more efficient method that skips the isolation step after carboxylation.The electrochemical performance of the resulting spinel lithium titanate is evaluated to be better than that of commercial competitors.The spinel lithium titanate negative electrode-based lithium-ion capacitor achieves a specific energy of 89.5 Wh kg À1 .These results demonstrate the success of efforts to find a feasible and affordable synthesis route to mitigate market risks.and mechanism are introduced into such a hybrid supercapacitor, it is also called Li-ion capacitor (LIC). [10,26,27]30][31] However, for the time being, most commercial supercapacitors still use activated carbon (AC) as their electrode material.The critical reason is the extreme price-performance ratio of AC, that is, not bad performance but extremely low price when compared to the new competitors.Spinel-type lithium titanate (Li 4 Ti 5 O 12 , LTO) is a well-known "zero-strain" material during Li-ion insertion/extraction. [32,33] Its theoretical capacity (175 mAh g À1 ) is not the highest among commonly used commercial LIBs anodes. [34]owever, the LTO, on the other hand, has several unique advantages, such as a stable and high operating voltage (1.55 V) versus Li/Li þ , no solid electrolyte interface film, no lithium dendrite, high Coulombic efficiency, excellent safety, long cycle life, resistance to over-charge and over-discharge, and tolerance to the low temperature below freezing point, etc. [35][36][37] The primary advantage of LTO is its intrinsic Li-ion diffusion coefficient, which is much higher than that of graphite anode, supporting enhanced fast charging-discharging performance. [38]So LTO is the optimal choice for high-power LIBs (negative electrodes), for example, the battery modules on urban buses fully charging in six minutes and operating in cold climates. [39,40]Similarly, LTO but not graphite carbon is an appropriate choice for LIC, as LIC emphasizes the material's rate performance to avoid converting a good supercapacitor into a mediocre battery, [7] and this viewpoint is taken into account at earliest research of LIC. [41]While LTO's electronic conductivity is restricted, it can be improved by reducing particle size, creating nanostructures, doping, and carbon-coating. [42,43]he commercial success of a material or product depends on not only its leading technical performance but also the cost, supply chain, technique compatibility, etc.This rule undoubtedly applies to LTO and LICs.During the past few years, extreme price variations and supply shortages of LIB-related commodities were the "black swan" for the LIB industry.Lithium carbonate (Li 2 CO 3 ) is undoubtedly a typical symbol of this issue.Due to Li 2 CO 3 being an essential raw material for many LIBs electrode materials (including LTO), [44,45] such a situation would pressure downstream products' (like LTO) supply and price.Even worse, these occurrences will probably become "gray rhino" in the future, considering the uncertainty of policy and geopolitics and the increasing energy storage demand of the end market.
As discussed above, LTO is a superior negative electrode material due to its specialties.However, its higher price than graphite carbon has already slowed the widespread commercialization of LTO.In recent years, chaos in the market, especially soaring prices and the tight supply of raw materials, further worsened the promotion and application of LTO.At worse, daily production and sales of LTO were seriously endangered for many manufacturers in China.Replacement/adjustment of raw materials and diversifying production routes for LTO are imminent needs and effective countermeasures.Furthermore, cost, supply chain resilience, compatibility with existing production lines, and environment friendliness should be considered comprehensively.Herein, we used cheaper and more readily available lithium hydroxide hydrate (LiOH•H 2 O) to prepare high-quality lithium carbonate.Additionally, we developed an optimized process to synthesize LTO from LiOH•H 2 O without isolating the intermediate product Li 2 CO 3 .This cost-effective alternative production approach can circumvent the barriers to widespread LTO usage, especially during market shocks.The obtained Li 2 CO 3 and LTO materials were characterized adequately, and the electrochemical performance of the LTO was investigated.Finally, LIC was assembled employing this LTO in combination with commercially available AC.The device exhibited a significantly high energy density and reserved power density simultaneously.
These results demonstrate the attractive potential of the alternative financially profitable synthesis route.It will promote the commercialization of LTO and LTO-based energy storage devices, including LIC, even during volatile market cycles.

Results and Discussion
][48] Among them, Li 2 CO 3 and LiOH are necessary raw materials for the battery especially the LIBs industry.They can be produced from salt lake brine, spodumene, and lepidolite via different technological processes. [47,48]As important commodities nowadays, spot prices and futures prices of Li 2 CO 3 and LiOH are complicated and depend not only on the exact Li element's content.Their prices are affected mainly by three aspects: 1) technological processes: for example, Li 2 CO 3 can be produced directly from salt lake brine, spodumene, and lepidolite without extra step, but LiOH can be produced directly from spodumene only or indirectly from brine/lepidolite with subsequent causticization process, and causticization will bring around 10 000 Chinese Yuan per ton (CNY ton À1 ) on the total cost; 2) upstream mineral resources' cost, involving resources' grades, place of production, shipping costs and duties, and being sensitive to climate, international politics, and market environment; 3) demand for downstream production: LIB positive electrode materials like LiCoO 2 , LiMn 2 O 4 , LiFePO 4 , and Li(Ni x Co y Mn 1ÀxÀy )O 2 (NCM, x = 3, 5, 6) usually utilize Li 2 CO 3 as the optimal raw material.However, LiOH is the favorable lithium source for nickel-rich layered oxide cathodes (e.g., NCM811).Lithium hydroxide's low melting point (462 °C) matches better with comparatively low sintering temperature for producing nickel-rich materials and using LiOH as raw material benefits nickel-rich material's specific capacity and cyclic performance. [49]he spot price of Li 2 CO 3 (battery grade, 99.5% purity, average price on China market, data supplier: Beijing Antaike Information Co., Ltd.) and lithium hydroxide hydrate (batterygrade coarse particle, 65.5% purity by LiOH, average price on China market, data supplier: Shanghai Metals Market) are demonstrated in Figure 1a and S1, Supporting Information.Due to the aforementioned factor 1 (causticization process) and factor 2 (most spodumene in China market is imported from Australia with a higher cost than salt lake brine from South America), usually Li 2 CO 3 is cheaper than the LiOH•H 2 O, such as in the first half year of 2017, from May 2018 to June 2019 (Figure 1a).From 2021, demand for LiFePO 4 increased quickly, mainly due to hot sales of electric vehicles in China (factor 3).Such demand, together with the COVID-19 pandemic (factor 2), pulled up the price of Li 2 CO 3 .The price spike lasted the following two years and reached nearly 600 000 CNY ton À1 (≈1500% of the lowest point) at the end of 2022.A similar situation was not alone, for example, the price jumped ≈4 times in the sixth months from November 2015 (Figure S2a, Supporting Information).Though the price of LiOH•H 2 O was affected by that of Li 2 CO 3 , other factors made their price variation not in sync.The price gap could be either positive or negative, and during the past 6.5 years, the price inversion (LiOH•H 2 O is cheaper) occurred six times, lasted summational around 34 months, and reached as high as 82000 CNY ton À1 (Figure 1b, areas filled with red shadow).During these time windows, replacing Li 2 CO 3 with cheaper LiOH•H 2 O is economically meaningful.Even if considering their Li element content difference and probable cost of chemically transforming LiOH into Li 2 CO 3 (≈8000 CNY ton À1 ), financially profitable periods still exist, which are partly shortened (Figure 1b, areas filled with red shadow but under the gray dash line).Figure S2a, Supporting Information, shows a similar price fluctuation trend using data from another platform, which collects more historical prices of Li 2 CO 3 .This dataset indicates another price inversion window from May 2015 to June 2016 (Figure S2b, Supporting Information).Notably, accompanied by a price spike, there is always a severe supply shortage crisis, for example, it is difficult to find plenty of Li 2 CO 3 spot in the New Year of 2016 and the first half of 2016.So diversifying the lithium resources, such as taking LiOH•H 2 O as a substitution, doesn't just release financial pressure.It secures the availability of Li, which will strengthen the safety and resilience of the supply chain for downstream products.
Lithium carbonate is also the most preferred lithium source raw material in the industrial manufacture of LTO.Its periodic price soars and supply shortages seriously affect LTO's production.After all, LTO has a smaller market share, so it is more sensitive to cost and more vulnerable to lose the fight for raw materials.Economically and scientifically, lithium hydroxide is a lithium source candidate for the synthesis of LTO. [50]wever, as a strong base, LiOH imposes anticorrosion requirements on the equipment and technical processes.Utilizing LiOH to prepare LTO directly on a large scale is challenging.Then, we investigated the indirect utilization of LiOH•H 2 O to synthesize LTO (i.e., transforming LiOH•H 2 O into Li 2 CO 3 ).The detailed procedures are described in the experimental section.As the raw material, lithium carbonate's quality is critical in controlling LTO's purity, crystallization, morphology, etc., which finally ensures LTO's electrochemical performance.Some critical characterization results of the as-prepared Li 2 CO 3 are presented below.
Scanning electron microscopy (SEM) images of white powder Li 2 CO 3 are shown in Figure 2a,b.The primary particles of lithium carbonate have dish-like forms with dimensions ranging from hundreds of nanometers to a few micrometers.Lithium carbonate secondary particles are a loose, irregular aggregation of primary particles, which have a dimension mainly distributed between 15 and 80 μm.The drying conditions and procedure dominate the secondary particles' size.It seems too large for the solid-phase reaction in LTO synthesis, but ball milling could solve the problem (Experimental Section).Figure 2c illustrates the X-ray diffraction (XRD) pattern of the obtained Li 2 CO 3 .It matches the standard PDF card well without other salt or oxide characteristic peaks. [51]The distinct peaks' narrow full widths at half maximum (FWHM) indicate perfect crystallization.The contents of other impurity elements in the obtained Li 2 CO 3 are listed in Table S1, Supporting Information, and the elements involved meet China's industry standard of battery-grade lithium carbonate. [52]These characteristics demonstrate adequate and appropriate conditions and procedures during the synthesis process.The morphology and particle dimension of TiO 2 that we used are investigated by SEM (Figure 2d,e).The primary particles of TiO 2 are smaller than 20 nm.Through high-energy ball milling, it could be mixed uniformly and has excellent nanoscale contact with primary particles of Li 2 CO 3 .This benefits the subsequent synthesis reaction of nanospinel lithium titanate.The dominant crystals of this TiO 2 are anatase, accompanied by very little rutile TiO 2 (Figure 2f ).Its Brunauer-Emmett-Teller (BET) specific surface area is 5.2 m 2 g À1 .Rutile TiO 2 is the stable crystal phase mainly applied in high-end white pigments, so cheaper anatase TiO 2 is the better choice here for cost control.
As the Experimental Section states, LTO can be synthesized from either isolate and dried Li 2 CO 3 powder or the reaction mixture.Both paths can produce a similar LTO product using distinct but optimized procedures.We choose the second option for the following research because it avoids separation and cleaning steps and has more significant potential for industrial-scale development.The XRD pattern (Figure 3d) of the as-prepared LTO reveals a spinel crystalline structure. [53]The narrow FWHM indicates the material's great purity and big crystal grain size.Two peaks near 0.2 and 1.9 μm in the particle size analysis result (Figure 3e) correspond to primary and secondary particle signals, as confirmed by SEM images in Figure 3a-c.The nanoscale dimension of primary particles partially ensures the material's good rate performance in a battery, and its secondary particle size meets the commercial standard for negative electrode materials.The morphology is adjustable, which can be controlled via existing abundant methods.The BET surface area of the obtained LTO is 5.52 m 2 g À1 .The Raman spectrum (Figure 3f ) clearly shows four characteristic peaks [53,54] in the low-wavenumber region that belongs to Li 4 Ti 5 O 12 .The Ti─O bond's IR peaks are also visible in the IR spectrum (Figure 3g). [55]There is also a trace of carbon element, which exists in the forms of C─H and C═O bonds.The presence of C element is confirmed by the elemental analysis (EA) result (Table S2, Supporting Information).However, its content is less than 0.2 wt%, which is too low to be found by thermogravimetric (TG) analysis (Figure 3h).The C element in LTO material should be derived from the polyvinyl alcohol employed in the production.We believe that such a small amount of C modification/doping helps improve the electrical conductivity of LTO. [42,43]owever, a more systematic investigation of the modification should be done separately.The possible metallic element impurities that may come from the synthesis process are also given in Table S2, Supporting Information.All have relatively low values.
The cyclic voltammetry (CV) curves of the obtained LTO are shown in Figure 3i.The locations of the peaks denote the potentials where lithium-ion insertion and extraction (primarily) occur.When the voltage scan speed increases from 0.1 to 0.3 mV s À1 , both peaks shift significantly away from the LTO's theoretical 1.55 V potential. [13]This is mainly due to electrochemical polarization.Such a result is comparable to others in the literature [43,56] that don't involve complex modifications or nanostructure design.
The prepared LTO is investigated by half cell in the potential range from 1.0 to 2.5 V versus Li/Li þ (Figure 4a).The current densities are set at 0.5, 1, 3, 5, and 10 C successively.Corresponding discharge capacities are 153.9,146.7, 128.4,108.7, and 62.4 mAh g À1 , respectively.The retentions of reversible capacity at 5 and 10 C are 70.6% and 40.5%, respectively.The discharge plateaus are 1.53 V for 0.5 C and 1.49 V for 1 C.Both are near the theoretical 1.55 V plateau of LTO. [36]The curved slopes in galvanostatic charge-discharge curves prove a certain portion of surface-controlled energy storage due to the nanosize effect, also called extrinsic pseudocapacitive. [57,58]The discharge plateau almost disappears when the current density reaches 10 C, which indicates a much-improved pseudocapacitive contribution at a higher rate. [59]Figure 4b shows the specific capacities for charging and discharging at different current rates for more cycles.The reversible capacities at each current density are stable enough.The Coulombic efficiencies are close to 100%, which is critical for high efficiency of energy utilization.We also investigated the performance of another commercial LTO material and plotted the results in Figure 4c,d.This referenced LTO material has ball-like secondary particles whose sizes are larger than 10 μm in diameter.Its discharge capacities are 144, 132, 105, 87, and 63 mAh g À1 at 0.5, 1, 3, 5, and 10 C, respectively.Except at 10 C, all results are lower than those of the LTO we prepared (Figure 4e).The commercial LTO, on the other hand, has a higher Coulombic efficiency and a longer discharge plateau at 10 C. It could be caused by morphology or an unspecified modification, which is enlightening for possible enhancement to our LTO material.The cycle stability of as-prepared LTO in LiPF 6 electrolyte is shown in Figure 4f.The current density is 87.5 mA g À1 , and the potential window is 1.0-2.5 V.The specific capacity is kept at 79% after 100 cycles and 71% after 400 cycles.The Coulombic efficiency is stable and almost 100%.These results reveal that the obtained LTO material exhibits remarkable electrochemical performance compared to pristine and unmodified LTO materials.
Figure 5a,b shows the SEM images of commercial AC we used.The irregular particles range from a few micrometers to slightly greater than ten micrometers.The most probable particle size, as determined by the particle size analysis (inset of Figure 5c), is 6.38 μm.D(10), D (50), and D(90) of the AC are 2.072, 5.636, and 11.274 μm, respectively.Some mesopores can be observed in the magnified SEM image (Figure 5b).However, the comprehensive aperture analysis shows that micropores comprise most of the total pore volume (Figure 5c).The BET specific surface area is 1478 m 2 g À1 , the BET average pore size is 1.84 nm, the Barrett-Joyner-Halenda (BJH) average pore diameter is 2.91 nm, and the whole pore volume is 0.048 cm 3 g À1 .The transmission electron microscopy (TEM) images (Figure S3, Supporting Information) demonstrate micropore patterns surrounded by amorphous carbon structures and a relatively localized crystallized structure (stacking of the graphene layers).The element contents of the AC obtained by EA are listed in Table S3, Supporting Information, with extremely low impurity contents tested by inductively coupled plasma mass spectrometry (ICP-MS) and spectrophotometry.
As an EDLC-type electrode material, the electrochemical properties of the AC in LiPF 6 electrolyte are also investigated using a half cell but not symmetric supercapacitor.Half-cell devices supply electrochemical potentials (vs.Li/Li þ ) information necessary for electrode matching in LIB/LIC but unnecessary in normal supercapacitors.The almost rectangular CV curves (Figure 5d) and triangular galvanostatic charge-discharge curves (Figure 5e) prove that the AC has nearly perfect and fast anion absorption/ desorption behavior over a wide potential range of 1.8-4.5 V vs. Li/Li þ .Such good EDLC-type behavior at the relatively high potential (vs.Li/Li þ ) makes it an ideal candidate as the positive electrode of LIC.The area surrounded by the CV curve varies slightly when the scan rate slows from 200 to 50 mV s À1 , indicating a good rate performance.The specific capacitances of AC in LiPF 6 electrolyte at different current densities are plotted in Figure 5f.The specific capacitance and specific capacity are convertible by mathematical calculation.So, the specific capacity of AC in the potential window of 2.5-4.5 V versus Li/Li þ is about 67 mAh g À1 when the current is 0.5 A g À1 .Besides, the specific capacitance retention of the AC is 89.7% of the initial value after 400 charge-discharge cycles at 1 A g À1 (Figure S4, Supporting Information).
The schematic of LIC with AC as the positive electrode, LTO as the negative electrode, and a LiPF 6 -containing organic electrolyte is shown in Figure 6a.During the charging process, the ACbased positive electrode's electrical potential increases due to the electronics' flow to the external power source, so the PF À anions adsorb onto the micropores surface inside the AC, forming an electrochemical double layer.At the other end, the electronics flow from a power source to the LTO electrode, and Li þ ions from the electrolyte intercalate into the LTO.The electrical potential of LTO decreases synchronously.Thus, the LIC device's voltage (positive electrode's potential minus negative electrode's potential) increases and electrical energy is stored inside the device. [10,57,58]The discharging process is the inverse of the above charging process (Figure 6a).To achieve equal electrical capacity between the positive and negative electrodes, the mass ratio (LTO electrode/ AC electrode) should ideally be around 2.7 (see Experimental Section).Compared with AC, LTO has a larger discharge capacity but decreases more rapidly with increasing current density.The EDLC-type electrode's actual specific capacity exhibition is significantly influenced by its actual working potential window within the device.Therefore, we tried to adjust the mass ratio and investigated LICs with the mass ratio (M AC /M LTO ) of 2.5, 3.0, and 3.5.Figure 6b shows the charge and discharge curves of the LIC with the mass ratio of 2.5.The curves' shape combines the AC electrode's triangular feature and the LTO electrode's plateaus.With rising current density, the plateau's contribution becomes less significant, making the LIC curves increasingly triangular.When the current density increases to 1.8 A g À1 (calculated by the LTO mass), the device completes one charging-discharging cycle with a maximum working voltage of 3.0 V in nine minutes without destroying curve symmetry.This demonstrates the device's high Coulombic efficiency at high current rates.
Figure 6c shows Ragone plots of the LICs compared to a symmetrical supercapacitor.The symmetrical supercapacitor AC//AC (working voltage 2.7 V) demonstrates a specific energy of 34.2 Wh kg À1 at a low specific power of 66.4 W kg À1 .All LICs in this work possess a much better energy storage capacity, even in the high-specific-power region (>1000 W kg À1 ).The LIC, with a mass ratio of 2.5, outperforms the others.Its specific energy reaches 89.5 Wh kg À1 when the specific power is 31 W kg À1 .When the device is operated at high current density, the specific power approaches 1550 W kg À1 while the specific energy is higher than 60 Wh kg À1 .Compared with LICs assembled by the same electrode materials, [60][61][62][63][64][65] the LIC (2.5) demonstrates the uppermost specific energy and leading specific power.The distinct advantage in the LIC's specific power, reported by Dsoke et al. [65] is realized by decreasing the M AC /M LTO from 4.17 to 0.72 and sacrificing specific energy.Some isolated data of specific energy or power released by commercial LIC manufacturers are shown in Figure 6c, too, and our LICs are still far ahead of them.
It is worth noting that the actual electrochemical behavior and potential window swing of both electrodes in LIC are complicated. [21]They will vary with the electrode mass ratio [65] and the current density, especially at high current densities.Specific technical methods can be employed to describe the device better under actual working conditions, such as introducing a third electrode (like a lithium electrode as the reference electrode) into the LIC device.These attempts will help us understand more about the device and improve its performance in the future.Last but not least, we believe that LICs are not simple replacements for LIBs or supercapacitors.Their best applications should be given after comprehensive requirement assessment, including energy, power, safety, lifespan, cost, self-discharge, thermal management, etc. [21] One crucial potential end-use of LICs is in regenerative braking energy storage for trains and heavy automotive, [21,66] neither battery nor supercapacitor solutions are able to meet requirements due to energy density limitation, heat dissipation problems, and overall implementation cost.This application is discussed in the literature and has been commercially developed by MAXWELL Technologies and CRRC Corporation Limited. [67]On the other hand, LICs, which are standalone or accompanied by batteries or solar cells, could supply robust, reliable, and cost-effective energy storage solutions. [21,66]These solutions can act as ride-through/backup powers for data storage systems, utility meters, and automated system controllers.Alternatively, they can optimize existing energy systems' performance, lifetime, and cost.9]

Conclusion
Affordable and stable prices, sufficient supply or stock, and outstanding performance are essential requirements for industrial battery materials.Diversification of the raw materials and the production technique guarantee the achievement of such goals.This work reviewed the market situations, conducted cost assessments, and experimentally demonstrated a feasible synthesis route to spinel lithium titanate from cheaper and more accessible lithium hydroxide hydrate.Battery-grade lithium carbonate, as the intermediate in production, can be isolated optionally and used for other purposes.Meanwhile, the alternative path skipping Li 2 CO 3 isolation but directly proceeding to LTO's synthesis is highly recommended due to its improved efficiency, potential to scale up, and same final production quality.The LTO produced has nanoscale primary particles with high purity and exhibits excellent electrochemical performance.By combining the high specific energy of the as-prepared LTO with the fast charging/ discharging merit of commercial AC, LIC has been developed that dramatically increases the specific energy of the energy storage device on the Ragone plots without sacrifices along the specific power direction.The synthesis route and application demonstration have shown success on the laboratory scale, and pilot-scale attempts will begin soon.[62][63][64][65] and data of some commercial LICs as references.

Experimental Section
Synthesis of Lithium Carbonate: An aqueous solution with 10% weight percent of LiOH was prepared with LiOH•H 2 O and deionized (DI) water, and the solution was stirred by a magnetic stirrer until it was transparent.Then, the solution was transferred into a three-necked flask with a thermometer.Two pipes for inputting gas into the system were fixed onto the flask.The end of the gas inlet pipe was placed at the bottom of the solution.CO 2 was fed into the solution at a rate of 1 L min À1 while the solution was stirred at a speed of <100 rpm.The temperature of the solution was kept between 25 and 35 °C.The white floc solid (Li 2 CO 3 ) would appear after several minutes and easily aggregate near the gas inlet.So, the tube's blockage should be avoided by adjusting the stirring speed, moving the tube's position, or increasing the gas flux accordingly.After about 45-60 min, the solution's pH decreased to 8-9, and the liquid and concrete mixture's weight reached 190-192% of the starting LiOH solution's weight.Then we stopped the CO 2 flow.The mixture was heated under stirring until its temperature reached 80-85 °C.The solid was separated by filtrating the hot mixture and then cleaned three times with hot water to remove dissolvable byproducts.The obtained Li 2 CO 3 solid was dried out at 120 °C in the oven for over 12 h.
Synthesis of Spinel Lithium Titanate: In this work, spinel lithium titanate was prepared in two ways.The first was to mix the Li 2 CO 3 mentioned earlier with TiO 2 in a molar ratio of Li/Ti = 22/25, DI water was added, and the mixture was stirred.Polyvinyl alcohol (1.5 wt% of the overall solids) was added as a surfactant, too.The suspension was ball milled at a 600 rpm speed for 5 h, and a slurry was obtained.The second method directly used the reaction mixture described in the earlier paragraph without conducting the solid-liquid separation procedure.TiO 2 was added into this suspension with the same Ti/Li molar ratio as in path one.DI water and polyvinyl alcohol could be used to adjust the viscosity and solid content of the mixture.The mixture was then ball milled at a 600 rpm speed for 5 h to obtain a homogenous slurry with smaller solid particles.The slurry was dried under 120 °C, calcined at 400 °C in the air for 4 h, and finally calcined at 850 °C in nitrogen atmosphere for another 6 h.The solid was ground after cooling to room temperature, and a white powder was obtained.The second path was recommended for simplification.
Fabrication of the Electrodes: For fabricating the battery-type electrode (as the positive electrode in a half-cell device and as the negative electrode in a LIC device), the active material (LTO) was mixed with super P and 5wt% PVDF solution (NMP as the solvent) to form a slurry.The mass ratio was LTO/super P/PVDF = 85/5/10.The slurry was blade coated onto the copper foil with the required thickness (or mass loading).This coated foil was placed into the oven and dried at 60 °C for 3 h.The temperature was increased to 150 °C and kept for another 1 h.Electrode chips with a 10 mm diameter were punched from this coated foil and weighed individually to obtain the active material's accurate mass.The loading density of active material (LTO) was ≈5 mg cm À2 .Finally, these electrode chips were dried thoroughly at 180 °C under high vacuum to desorb water molecules or other gas molecules.They were stored in the glovebox.The EDLC-type electrode served as the positive electrode in a half-cell device and LIC device.In a symmetric supercapacitor, "positive" and "negative" electrodes were interchangeable; both were EDLC-type electrodes.For fabricating this electrode, AC (YP50) powder was mixed with super P and PTFE (85:5:10).A little ethanol was added and milled in a mortar until obtaining a uniform plasticine-like mixture.Then, the mixture was rolled into thin sheets.Their thickness/loading density was adjusted on demand.The sheet was then hot pressed onto the aluminum foil with super P coating. 1 cm diameter electrodes were punched from such a composite film and weighed.The electrodes were dried at 120 °C for 6 h and further dried and desorbed at 180 °C overnight with high-vacuum assistance.These electrodes were transferred into the glovebox for storage until the device assembly.
Fabrication of the Devices: All the devices' assembly processes were finished in the glovebox with argon atmosphere.Two identical AC (YP50) electrodes were soaked in the electrolyte for a few minutes.Then they were assembled with polymer separator and electrolyte into the stain steel battery cases (coin cell model 2032).All other procedures followed the standard requirements of coin cell assembly.If one EDLC-type electrode was replaced by a lithium metal chip that acted as the counter and reference electrode, a half-cell type device was obtained.The combination of battery-type electrode (i.e., the LTO electrode in this work) and lithium chip was another half-cell device.LTO half-cell devices would discharge-charge at 0.2 C (1 C = 175 mAh g À1 ) for two cycles before conducting other electrochemical measurements.Half-cell devices were used to investigate materials' electrochemical performance in a specific potential window versus Li/Li þ .Finally, the AC electrode and LTO electrode were assembled and worked as the positive and negative electrodes of LIC.The separator and organic electrolyte in the above devices were identical and described in Supporting Information.The effective mass of active materials in the AC electrode and LTO electrode can be adjusted for the device's best performance.The positive electrode's electrical capacity was equal to that of the negative electrode.Theoretically, the mass ratio of the positive and negative electrodes should be reciprocal to the specific capacity ratio of the two electrodes.The discharge specific capacity of LTO was 150 mAh g À1 at 87.5 mA g À1 and ≈135 mAh g À1 at 380 mA g À1 .The specific capacity of AC was about 70 mAh g À1 at 100 mA g À1 and ≈67 mAh g À1 at 380 mA g À1 , if the potential window was 2 V (34 mAh g À1 , 1 V potential window; 50 mAh g À1 , 1.5 V potential window).Due to the plateau region contributing the main part of the LTO's specific capacity in a narrow potential window, we distributed a 1.5 V potential window to the AC electrode in LIC, so the mass ratio should be 2.7 (135/50) or less.If trying to take advantage of LTO's specific capacity as much as possible, the mass ratio should be increased and a smaller potential window should be distributed to the AC electrode.The limit of mass ratio should be 4.0 (135/34).Another factor should be considered: when the current density increases, LTO's specific capacity decreases, and this makes the optimized mass ratio decrease.Based on the above simplified estimation and deviation between LIC's actual and designed running states, we experimentally investigated the mass ratios of 2.5, 3.0, and 3.5.All devices were aged overnight before testing.
Characterizations: SEM images were obtained on the FEI NanoSem 430 field-emission scanning electron microscope with an accelerating voltage between 5 and 20 kV.XRD measurements were carried out on a Rigaku D/Max-2500 diffractometer with a Cu Kα radiation line.ICP-MS was obtained by the Thermo Elemental X7 Series ICP-MS.Particle size analysis was carried out using the Malvern Mastersizer 2000 particle size analyzer.Raman spectra were performed on a Renishaw inVia Raman spectrometer using a 514.5 nm excitation laser.The Mettler Toledo thermogravimetric analyzer did TG analysis at 10°min À1 under the O 2 atmosphere.The nitrogen adsorption-desorption analysis was conducted at 77 K on a Micromeritics ASAP 2020 apparatus.The surface area was calculated using the BET method with data in the relative pressure range between 0.05 and 0.3.The pore size distribution was computed using the nitrogen adsorption data and the non-local density functional theory (NL-DFT) method with a slit pore model.TEM was carried out on a JEOL TEM-2100 transmission electron microscope operated at 200 kV.EA was carried out using an Elementar Vario Micro cube analyzer.IR spectrum was obtained by the Bruker Tensor 27 fourier-transform infrared spectroscopy (FT-IR) spectrometer.Inductively coupled plasma atomic emission spectroscopy (ICP-AES) was carried out on the Hitachi 180-80 Polarized Zeeman Atomic Absorption Spectrometer.The content of element Cl in AC was determined by the titration-spectrophotometry method, and the AC was dissolved by microwave-assisted digestion.
Electrochemical Measurements: All the electrochemical tests were carried out at room temperature.Galvanostatic charge-discharge measurements of all devices were conducted using the battery test system LAND CT2001A (Wuhan LAND Electronics Company).CV studies were performed with different voltage scan rates and electrochemical potential windows on the electrochemical analyzer LK98BII (LANLIKE).The specific capacities of materials were recorded directly by the test system's software.It can be calculated according to the following formula:Q ¼ ∫ t 2 t 1 Idt, where Q (mAh g À1 ) is the specific capacity of the electrode material, I (A g À1 ) is the current density, t (s) is the time, and this formula is suitable for both charge and discharge processes.The specific capacitance C s of AC in half-cell devices was calculated according to the following equation: C s ¼ Q=V, where Q (mAh g À1 ) is the specific capacity, V (V) is the potential difference (or the width) of the electrochemical window.The specific energy of symmetric supercapacitors and LICs was obtained by the following formula: E ¼ ∫ q 2 q 1 Vdq=M ¼ ∫ t 2 t 1 IVdt=M, where E (Wh kg À1 ) is the device's specific energy (only considering the active materials' mass), q (Ah) is the discharge capacity of the device, V (V) is the voltage of the device, M (kg) is the total mass of active materials in two electrodes, I (A) is the discharge current, and t (s) is the time of the discharge process.The specific power of the device was calculated by the equation: P ¼ E=Δt, where P (W kg À1 ) is the specific power of the device, E (Wh kg À1 ) is the specific energy of the device, and Δt is the whole discharge time.

Figure 1 .
Figure 1.a) The historical spot prices of lithium carbonate and lithium hydroxide hydrate from January 2017 to August 2023.All the prices were supplied by the financial database WIND.b) The price gap of lithium carbonate and lithium hydroxide hydrate in the same period in (a), which is defined as lithium hydroxide hydrate's price minus lithium carbonate's price.

Figure 2 .
Figure 2. a,b) SEM images of the as-prepared lithium carbonate, scale bars are 20 μm in (a) and 5 μm in (b).c) XRD spectrum of the obtained Li 2 CO 3 .d,e) SEM images of the TiO 2 that we used, and the scale bars are 5 μm in (d) and 500 nm in (e).The XRD spectrum of the TiO 2 is shown in (f ) with standard spectra.

Figure 3 .
Figure 3. a-c) SEM images of the as-prepared LTO with different magnetization levels.Scale bars are 10 μm in (a), 2 μm in (b), and 300 nm in (c), respectively.d) XRD spectrum, e) particle size distribution, f ) Raman spectrum, g) IR spectrum, and h) TG curve of the obtained LTO.i) CV curves of the obtained LTO material at different scan rates in the potential range of 0.9-2.4V versus Li/Li þ .

Figure 4 .
Figure 4. a) Charge-discharge curves of the as-prepared LTO at different current densities between 0.5 and 10 C. b) Rate performance and corresponding Coulombic efficiency of the LTO at different current densities in the potential window of 1.0-2.5 V versus Li/Li þ .c) Charge-discharge curves and d) rate performance and Coulombic efficiency of the referenced commercial LTO are measured under the same conditions.Inset of (d) shows the SEM image of this commercial LTO material with a scale bar of 2 μm.e) Comparison of the specific capacities of two LTO materials at different current densities.f ) Cycle stability of the as-prepared LTO at 0.5 C with high Coulombic efficiency.

Figure 5 .
Figure 5. a,b) SEM images of AC.Scale bars are 10 μm for (a), 2 μm for the inset of (a), and 200 nm for (b).c) The pore size distribution of the AC is measured and calculated based on the NL-DFT model.The inset shows the particle size distribution of the AC.d) CV curves of the AC at different scan rates in a wide potential window.e) Galvanostatic charge-discharge curves of AC at different current densities.f ) The specific capacitance of the AC obtained from a half-cell device at different current densities.

Figure 6 .
Figure 6.a) Schematic for the charge and discharge processes of the LIC device with AC and LTO as electrode materials.b) Typical charge-discharge curves of the LIC device.c) The Ragone plots of LICs with different mass ratios (M AC /M LTO = 2.5, 3.0, and 3.5), the AC-based symmetrical supercapacitor, other LICs based on LTO and AC in the literature,[60][61][62][63][64][65] and data of some commercial LICs as references.