Solid‐State Lithium Batteries: Bipolar Design, Fabrication, and Electrochemistry

Advanced parametrization for the production of high-energy solid-state lithium pouch cells containing polymer electrolytes

Lithium batteries with solid-state electrolytes are an appealing alternative to state-of-the-art non-aqueous lithium-ion batteries with liquid electrolytes because of safety and energy aspects. However, engineering development at the cell level for lithium batteries with solid-state electrolytes is limited. Here, to advance this aspect and produce high-energy lithium cells, we introduce a cell design based on advanced parametrization of microstructural and architectural parameters of electrode and electrolyte components. To validate the cell design proposed, we assemble and test (applying a stack pressure of 3.74 MPa at 45 °C) 10-layer and 4-layer solid-state lithium pouch cells with a solid polymer electrolyte, resulting in an initial specific energy of 280 Wh kg-1 (corresponding to an energy density of 600 Wh L-1) and 310 Wh kg-1 (corresponding to an energy density of 650 Wh L-1) respectively.

Bipolar Textile Composite Electrodes Enabling Flexible Tandem Solid-State Lithium Metal Batteries

A majority of flexible and wearable electronics require high operational voltage that is conventionally achieved by serial connection of battery unit cells using external wires. However, this inevitably decreases the energy density of the battery module and may cause additional safety hazards. Herein, a bipolar textile composite electrode (BTCE) that enables internal tandem-stacking configuration to yield high-voltage (6 to 12 V class) solid-state lithium metal batteries (SSLMBs) is reported. BTCE is comprised of a nickel-coated poly(ethylene terephthalate) fabric (NiPET) core layer, a cathode coated on one side of the NiPET, and a Li metal anode coated on the other side of the NiPET. Stacking BTCEs with solid-state electrolytes alternatively leads to the extension of output voltage and decreased usage of inert package materials, which in turn significantly boosts the energy density of the battery. More importantly, the BTCE-based SSLMB possesses remarkable capacity retention per cycle of over 99.98% over cycling. The composite structure of BTCE also enables outstanding flexibility; the battery keeps stable charge/discharge characteristics over thousands of bending and folding. BTCE shows great promise for future safe, high-energy-density, and flexible SSLMBs for a wide range of flexible and wearable electronics.

Bridging the gap between academic research and industrial development in advanced all-solid-state lithium-sulfur batteries

The energy storage and vehicle industries are heavily investing in advancing all-solid-state batteries to overcome critical limitations in existing liquid electrolyte-based lithium-ion batteries, specifically focusing on mitigating fire hazards and improving energy density. All-solid-state lithium-sulfur batteries (ASSLSBs), featuring earth-abundant sulfur cathodes, high-capacity metallic lithium anodes, and non-flammable solid electrolytes, hold significant promise. Despite these appealing advantages, persistent challenges like sluggish sulfur redox kinetics, lithium metal failure, solid electrolyte degradation, and manufacturing complexities hinder their practical use. To facilitate the transition of these technologies to an industrial scale, bridging the gap between fundamental scientific research and applied R&D activities is crucial. Our review will address the inherent challenges in cell chemistries within ASSLSBs, explore advanced characterization techniques, and delve into innovative cell structure designs. Furthermore, we will provide an overview of the recent trends in R&D and investment activities from both academia and industry. Building on the fundamental understandings and significant progress that has been made thus far, our objective is to motivate the battery community to advance ASSLSBs in a practical direction and propel the industrialized process.

From Liquid to Solid-State Batteries: Li-Rich Mn-Based Layered Oxides as Emerging Cathodes with High Energy Density

Li-rich Mn-based (LRMO) cathode materials have attracted widespread attention due to their high specific capacity, energy density, and cost-effectiveness. However, challenges such as poor cycling stability, voltage deca,y and oxygen escape limit their commercial application in liquid Li-ion batteries. Consequently, there is a growing interest in the development of safe and resilient all-solid-state batteries (ASSBs), driven by their remarkable safety features and superior energy density. ASSBs based on LRMO cathodes offer distinct advantages over conventional liquid Li-ion batteries, including long-term cycle stability, thermal and wider electrochemical windows stability, as well as the prevention of transition metal dissolution. This review aims to recapitulate the challenges and fundamental understanding associated with the application of LRMO cathodes in ASSBs. Additionally, it proposes the mechanisms of interfacial mechanical and chemical instability, introduces noteworthy strategies to enhance oxygen redox reversibility, enhances high-voltage interfacial stability, and optimizes Li+ transfer kinetics. Furthermore, it suggests potential research approaches to facilitate the large-scale implementation of LRMO cathodes in ASSBs.

Toward Sustainable All Solid-State Li-Metal Batteries: Perspectives on Battery Technology and Recycling Processes

Lithium (Li)-based batteries are gradually evolving from the liquid to the solid state in terms of safety and energy density, where all solid-state Li-metal batteries (ASSLMBs) are considered the most promising candidates. This is demonstrated by the Bluecar electric vehicle produced by the Bolloré Group, which is utilized in car-sharing services in several cities worldwide. Despite impressive progress in the development of ASSLMBs, their avenues for recycling them remain underexplored, and combined with the current explosion of spent Li-ion batteries, they should attract widespread interest from academia and industry. Here, the potential challenges of recycling ASSLMBs as compared to Li-ion batteries are analyzed and the current progress and prospects for recycling ASSLMBs are summarized and analyzed. Drawing on the lessons learned from Li-ion battery recycling, it is important to design sustainable recycling technologies before ASSLMBs gain widespread market adoption. A battery-recycling-oriented design is also highlighted for ASSLMBs to promote the recycling rate and maximize profitability. Finally, future research directions, challenges, and prospects are outlined to provide strategies for achieving sustainable development of ASSLMBs.

Robust and Manufacturable Lithium Lanthanum Titanate-Based Solid-State Electrolyte Thin Films Deposited in Open Air

State-of-the-art solid-state electrolytes (SSEs) are limited in their energy density and processability based on thick, brittle pellets, which are generally hot pressed in vacuum over the course of several hours. We report on a high-throughput, open-air process for printable thin-film ceramic SSEs in a remarkable one-minute time frame using a lithium lanthanum titanium oxide (LLTO)-based SSE that we refer to as robust LLTO (R-LLTO). Powder XRD analysis revealed that the main phase of R-LLTO is polycrystalline LLTO, accompanied by selectively retained crystalline precursor phases. R-LLTO is highly dense and closely matched to the stoichiometry of LLTO with some heterogeneity throughout the film. A minimal presence of lithium carbonate is identified despite processing fully in ambient conditions. The LLTO films exhibit remarkable mechanical properties, demonstrating both flexibility with a low modulus of ∼35 GPa and a high fracture toughness of >2.0 . We attribute this mechanical robustness to several factors, including grain boundary strengthening, the presence of precursor crystalline phases, and a decrease in crystallinity or ordering caused by ultrafast processing. The creation of R-LLTO-a ceramic material with elastic properties that are closer to polymers with higher fracture toughness-enables new possibilities for the design of robust solid-state batteries.

Monolithically-stacked thin-film solid-state batteries

The power capability of Li-ion batteries has become increasingly limiting for the electrification of transport on land and in the air. The specific power of Li-ion batteries is restricted to a few thousand W kg^-1 due to the required cathode thickness of a few tens of micrometers. We present a design of monolithically-stacked thin-film cells that has the potential to increase the power ten-fold. We demonstrate an experimental proof-of-concept consisting of two monolithically stacked thin-film cells. Each cell consists of a silicon anode, a solid-oxide electrolyte, and a lithium cobalt oxide cathode. The battery can be cycled for more than 300 cycles between 6 and 8 V. Using a thermo-electric model, we predict that stacked thin-film batteries can achieve specific energies >250 Wh kg^-1 at C-rates above 60, resulting in a specific power of tens of kW kg^-1 needed for high-end applications such as drones, robots, and electric vertical take-off and landing aircrafts.

Progress and Prospects of Inorganic Solid-State Electrolyte-Based All-Solid-State Pouch Cells

All-solid-state batteries have piqued global research interest because of their unprecedented safety and high energy density. Significant advances have been made in achieving high room-temperature ionic conductivity and good air stability of solid-state electrolytes (SSEs), mitigating the challenges at the electrode-electrolyte interface, and developing feasible manufacturing processes. Along with the advances in fundamental study, all-solid-state pouch cells using inorganic SSEs have been widely demonstrated, revealing their immense potential for industrialization. This review provides an overview of inorganic all-solid-state pouch cells, focusing on ultrathin SSE membranes, sheet-type thick solid-state electrodes, and bipolar stacking. Moreover, several critical parameters directly influencing the energy density of all-solid-state Li-ion and lithium-sulfur pouch cells are outlined. Finally, perspectives on all-solid-state pouch cells are provided and specific metrics to meet certain energy density targets are specified. This review looks to facilitate the development of inorganic all-solid-state pouch cells with high energy density and excellent safety.

Lithium Difluorophosphate: A Boon for High Voltage Li Ion Batteries and a Bane for High Thermal Stability/Low Toxicity: Towards Synergistic Dual Additives to Circumvent this Dilemma

The specific energy/energy density of state-of-the-art (SOTA) Li-ion batteries can be increased by raising the upper charge voltage. However, instability of SOTA cathodes (i. e., LiNi_y Co_x Mn_y O₂ ; x+y+z=1; NCM) triggers electrode crosstalk through enhanced transition metal (TM) dissolution and contributes to severe capacity fade; in the worst case, to a sudden death ("roll-over failure"). Lithium difluorophosphate (LiDFP) as electrolyte additive is able to boost high voltage performance by scavenging dissolved TMs. However, LiDFP is chemically unstable and rapidly decomposes to toxic (oligo)organofluorophosphates (OFPs) at elevated temperatures; a process that can be precisely analyzed by means of high-performance liquid chromatography-high resolution mass spectroscopy. The toxicity of LiDFP can be proven by the well-known acetylcholinesterase inhibition test. Interestingly, although fluoroethylene carbonate (FEC) is inappropriate for high voltage applications as a single electrolyte additive due to rollover failure, it is able to suppress formation of toxic OFPs. Based on this, a synergistic LiDFP/FEC dual-additive approach is suggested in this work, showing characteristic benefits of both individual additives (good capacity retention at high voltage in the presence of LiDFP and decreased OFP formation/toxicity induced by FEC).

Stacking Faults Assist Lithium-Ion Conduction in a Halide-Based Superionic Conductor

In the pursuit of urgently needed, energy dense solid-state batteries for electric vehicle and portable electronics applications, halide solid electrolytes offer a promising path forward with exceptional compatibility against high-voltage oxide electrodes, tunable ionic conductivities, and facile processing. For this family of compounds, synthesis protocols strongly affect cation site disorder and modulate Li⁺ mobility. In this work, we reveal the presence of a high concentration of stacking faults in the superionic conductor Li₃YCl₆ and demonstrate a method of controlling its Li⁺ conductivity by tuning the defect concentration with synthesis and heat treatments at select temperatures. Leveraging complementary insights from variable temperature synchrotron X-ray diffraction, neutron diffraction, cryogenic transmission electron microscopy, solid-state nuclear magnetic resonance, density functional theory, and electrochemical impedance spectroscopy, we identify the nature of planar defects and the role of nonstoichiometry in lowering Li⁺ migration barriers and increasing Li site connectivity in mechanochemically synthesized Li₃YCl₆. We harness paramagnetic relaxation enhancement to enable ⁸⁹Y solid-state NMR and directly contrast the Y cation site disorder resulting from different preparation methods, demonstrating a potent tool for other researchers studying Y-containing compositions. With heat treatments at temperatures as low as 333 K (60 °C), we decrease the concentration of planar defects, demonstrating a simple method for tuning the Li⁺ conductivity. Findings from this work are expected to be generalizable to other halide solid electrolyte candidates and provide an improved understanding of defect-enabled Li⁺ conduction in this class of Li-ion conductors.

Single-Ion versus Dual-Ion Conducting Electrolytes: The Relevance of Concentration Polarization in Solid-State Batteries

Lithium batteries with solid polymer electrolytes (SPEs) and mobile ions are prone to mass transport limitations, that is, concentration polarization, creating a concentration gradient with Li⁺-ion (and counter-anion) depletion toward the respective electrode, as can be electrochemically observed in, for example, symmetric Li||Li cells and confirmed by Sand and diffusion equations. The effect of immobile anions is systematically investigated in this work. Therefore, network-based SPEs are synthesized with either mobile (dual-ion conduction) or immobile anions (single-ion conduction) and proved via solvation tests and nuclear magnetic resonance spectroscopy. It is shown that the SPE with immobile anions does not suffer from concentration polarization, thus disagreeing with Sand and diffusion assumptions, consequently suggesting single-ion (Li⁺) transport via migration instead. Nevertheless, the practical relevance of single-ion conduction can be debated. Under practical conditions, that is, below the limiting current, the concentration polarization is generally not pronounced with DIC-based electrolytes, rendering the beneficial effect of SIC redundant and DIC a better choice due to better kinetical aspects under these conditions. Also, the observed dendritic Li in both electrolytes questions a relevant impact of mass transport on its formation, at least in SPEs.

High-Power Bipolar Solid-State Batteries Enabled by In-Situ-Formed Ionogels for Vehicle Applications

Employing solid electrolytes (SEs) for lithium-ion batteries can boost the battery tolerance under abusive conditions and enable the implementation of bipolar cell stacking, leading to higher cell energy and power density as well as simplified thermal management. In this context, a bipolar solid-state battery (SSB) has received ever-increasing attention in recent years. However, poor solid-solid interfacial contact within the bipolar SSB deteriorates the battery power capability, representing a technical challenge for vehicle applications. In this work, a bipolar SSB pouch cell with two cell units connected in series is demonstrated without any short circuit or current leakage. With the assistance of an in-situ-formed nonflammable ionogel at particle-to-particle interfaces, the constructed bipolar cell manifests superior power capability and can meet the engineering cold crank requirements in 0, -10, and -18 °C environments. Furthermore, the excellent tolerance of the ionogel-introduced bipolar SSB under abusive conditions was proved by folding, cutting, and burning the cells. The above salient features suggested that the developed strategy herein holds promise to advance the next-generation high-performance SSBs.

Nominally stoichiometric Na3(WxSixSb1-2x)S4 as a superionic solid electrolyte

Na3MX4 (M = P, Sb and X = S, Se) and its doped analogues are considered as a promising material in room-temperature (RT) Na+-conducting solid electrolytes. Herein, we first report...

Configuring solid-state batteries to power electric vehicles: a deliberation on technology, chemistry and energy

Solid-state batteries (SSBs) have been widely regarded as a promising electrochemical energy storage technology to power electric vehicles (EVs) that raise battery safety and energy/power densities as kernel metrics to achieve high-safety, long-range and fast-charge operations. Governments around the world have set ambitious yet imperative goals on battery energy density; however, sluggish charge transport and challenging processing routes of SSBs raise doubts of whether they have the possibility to meet such targets. In this contribution, the battery development roadmap of China is set as the guideline to direct how material chemistries and processing parameters of SSBs need to be optimized to fulfill the requirements of battery energy density. Starting with the identification of bipolar cell configurations in SSBs, the blade cell dimension is then selected as an emerging cell format to clarify weight breakdown of a solid NCM523||Li cell. Quantifying energy densities of SSBs by varying key cell parameters reveals the importance of active material content, cathode layer thickness and solid-electrolyte-separator thickness, whereas the thicknesses of the lithium metal anode and bipolar current collector have mild impacts. Even in the pushing conditions (200 μm for the cathode layer and 20 μm for the solid electrolyte separator), high-nickel ternary (NCM) cathodes hardly meet the expectation of the battery development roadmap in terms of gravimetric energy density at a cell level, while lithium- and manganese-rich ternary (LM-NCM) and sulfur cathodes are feasible. In particular, solid lithium-sulfur batteries, which exhibit exciting gravimetric energy density yet inferior volumetric energy density, need to be well-positioned to adapt diverse application scenarios. This analysis unambiguously defines promising battery chemistries and establishes how key parameters of SSBs can be tailored to cooperatively follow the stringent targets of future battery development.

Quasi-Solid-State Lithium Metal Batteries Using the LiNi0.8Co0.1Mn0.1O2-Li1+xAlxTi2-x(PO4)3 Composite Positive Electrode

NASICON-type Li_1+xAl_xTi_2-x(PO₄)₃ (LATP) is a promising solid electrolyte (SE) candidate for next-generation solid-state batteries. However, its use in solid-state composite electrodes is inhibited by its stiffness, which results in poor interparticle contact unless high-temperature treatments are applied. The poor LATP-LATP and LATP-active material in the positive electrode (cathode) composite produced at ambient temperature yield poor ionic conductivity, impeding the electrode's performance. Herein, we focus on the optimization of the electrochemical performance of LiNi_0.8Co_0.1Mn_0.1O₂ (NCM₈₁₁)-LATP composite electrodes made by tape casting, taking advantage of a small fraction of an ionic liquid electrolyte (ILE) filling the composite cathode porosity. The incorporated LATP particles are found to closely surround the large NCM₈₁₁ secondary particles, partially filling the composite electrode pores and resulting in a porosity reduction from 37 vol % (NCM₈₁₁ only) to 32 vol % (NCM₈₁₁-LATP). After filling up the majority of the electrode porosity with ILE, the NCM₈₁₁-LATP composite electrodes offer improved capacity retention upon both long-term cycling tests (>99.3% after 200 cycles) and high-rate tests (>70% at 2 C-rate), due to the more stable LATP|NCM₈₁₁ interface, and facilitated Li⁺ diffusion in the composite electrode bulk. Results obtained from proof-of-concepts monopolar (3.0-4.3 V) and bipolar-stacked (6.0-8.6 V) cells are reported.

Area Oversizing of Lithium Metal Electrodes in Solid-State Batteries: Relevance for Overvoltage and thus Performance?

Systematic and systemic research and development of solid electrolytes for lithium batteries requires a reliable and reproducible benchmark cell system. Therefore, factors relevant for performance, such as temperature, voltage operation range, or specific current, should be defined and reported. However, performance can also be sensitive to apparently inconspicuous and overlooked factors, such as area oversizing of the lithium electrode and the solid electrolyte membrane (relative to the cathode area). In this study, area oversizing is found to diminish polarization and improves the performance in LiNi0.6 Mn0.2 Co0.2 O2 (NMC622)||Li cells, with a more pronounced effect under kinetically harsh conditions (e. g., low temperature and/or high current density). For validity reasons, the polarization behavior is also investigated in Li||Li symmetric cells. Given the mathematical conformity of the characteristic overvoltage behavior with the Sand's equation, the beneficial effect is attributed to lower depletion of Li ions at the electrode/electrolyte interface. In this regard, the highest possible effect of area oversizing on the performance is discussed, that is when the accompanied decrease in current density and overvoltage overcomes the Sand's threshold limit. This scenario entirely prevents the capacity decay attributable to Li+ depletion and is in line with the mathematically predicted values.

Macromolecular Design of Lithium Conductive Polymer as Electrolyte for Solid-State Lithium Batteries

In the development of solid-state lithium batteries, solid polymer electrolyte (SPE) has drawn extensive concerns for its thermal and chemical stability, low density, and good processability. Especially SPE efficiently suppresses the formation of lithium dendrite and promotes battery safety. However, most of SPE is derived from the matrix with simple functional group, which suffers from low ionic conductivity, reduced mechanical properties after conductivity modification, bad electrochemical stability, and low lithium-ion transference number. Appling macromolecular design with multiple functional groups to polymer matrix is accepted as a strategy to solve the problems of SPE fundamentally. In this review, macromolecular design based on lithium conducting groups is summarized including copolymerization, network construction, and grafting. Meanwhile, the construction of single-ion conductor polymer is also focused herein. Moreover, synergistic effects between the designed matrix, lithium salt, and fillers are reviewed with the objective to further improve the performance of SPE. At last, future studies on macromolecular design are proposed in the development of SPE for solid-state batteries with high energy density and durability.

Passivation of the Cathode-Electrolyte Interface for 5 V-Class All-Solid-State Batteries

An all-solid-state battery is a potentially superior alternative to a state-of-the-art lithium-ion battery owing to its merits in abuse tolerance, packaging, energy density, and operable temperature ranges. In this work, a 5 V-class spinel LiNi_0.5Mn_1.5O₄ (LNMO) cathode is targeted to combine with a high-ionic-conductivity Li₆PS₅Cl (LPSCl) solid electrolyte for developing high-performance all-solid-state batteries. Aiming to passivate and stabilize the LNMO-LPSCl interface and suppress the unfavorable side reactions such as the continuous chemical/electrochemical decomposition of the solid electrolyte, oxide materials including LiNbO₃, Li₃PO₄, and Li₄Ti₅O₁₂ are rationally applied to decorate the surface of pristine LNMO particles with various amounts through a wet-chemistry approach. Electrochemical characterization demonstrates that the composite cathode consisting of 8 wt % LiNbO₃-coated LNMO and LPSCl in a weight ratio of 70:30 delivers the best electrochemical performance with an initial discharge capacity of 115 mA h g^-1 and a reversible discharge capacity of 80 mA h g^-1 at the 20th cycle, suggesting that interfacial passivation is an effective strategy to ensure the operation of 5 V-class all-solid-state batteries.

Elimination of "Voltage Noise" of Poly (Ethylene Oxide)-Based Solid Electrolytes in High-Voltage Lithium Batteries: Linear versus Network Polymers

Frequently, poly(ethylene oxide) (PEO)-based solid polymer electrolytes (SPEs) reveal a failure with high-voltage electrodes, e.g. LiNi_0.6Mn_0.2Co_0.2O₂ in lithium metal batteries, which can be monitored as an arbitrary appearance of a “voltage noise” during charge and can be attributed to Li dendrite-induced cell micro short circuits. This failure behavior disappears when incorporating linear PEO-based SPE in a semi-interpenetrating network (s-IPN) and even enables an adequate charge/discharge cycling performance at 40°C. An impact of any electrolyte oxidation reactions on the performance difference can be excluded, as both SPEs reveal similar (high) bulk oxidation onset potentials of ≈4.6 V versus Li|Li⁺. Instead, improved mechanical properties of the SPE, as revealed by compression tests, are assumed to be determining, as they mechanically better withstand Li dendrite penetration and better maintain the distance of the two electrodes, both rendering cell shorts less likely.

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PEO-based solid polymer electrolytes (SPEs) are stable up to 4.6 V versus Li|Li⁺

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But, linear PEO-based SPE results in a “voltage noise-failure” in NMC‖Li cells

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Failure disappears when PEO is incorporated in a semi-interpenetrating network

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Charge/discharge cycling possible even at 40°C in NMC622‖Li cells

Review of the Design of Current Collectors for Improving the Battery Performance in Lithium-Ion and Post-Lithium-Ion Batteries

Current collectors (CCs) are an important and indispensable constituent of lithium-ion batteries (LIBs) and other batteries. CCs serve a vital bridge function in supporting active materials such as cathode and anode materials, binders, and conductive additives, as well as electrochemically connecting the overall structure of anodes and cathodes with an external circuit. Recently, various factors of CCs such as the thickness, hardness, compositions, coating layers, and structures have been modified to improve aspects of battery performance such as the charge/discharge cyclability, energy density, and the rate performance of a cell. In this paper, the details of interesting and useful attempts of preparing CCs for high battery performance in lithium-ion and post-lithium-ion batteries are reviewed. The advantages and disadvantages of these attempts are discussed.