Defect Strategy in Solid-State Lithium Batteries

Solid-state lithium batteries (SSLBs) have great development prospects in high-security new energy fields, but face major challenges such as poor charge transfer kinetics, high interface impedance, and unsatisfactory cycle stability. Defect engineering is an effective method to regulate the composition and structure of electrodes and electrolytes, which plays a crucial role in dominating physical and electrochemical performance. It is necessary to summarize the recent advances regarding defect engineering in SSLBs and analyze the mechanism, thus inspiring future work. This review systematically summarizes the role of defects in providing storage sites/active sites, promoting ion diffusion and charge transport of electrodes, and improving structural stability and ionic conductivity of solid-state electrolytes. The defects greatly affect the electronic structure, chemical bond strength and charge transport process of the electrodes and solid-state electrolytes to determine their electrochemical performance and stability. Then, this review presents common defect fabrication methods and the specific role mechanism of defects in electrodes and solid-state electrolytes. At last, challenges and perspectives of defect strategies in high-performance SSLBs are proposed to guide future research.

Gradient Nitrogen Doping in the Garnet Electrolyte for Highly Efficient Solid-State-Electrolyte/Li Interface by N2 Plasma

Solid-state lithium batteries (SSBs) have been widely researched as next-generation energy storage technologies due to their high energy density and high safety. However, lithium dendrite growth through the solid electrolyte usually results from the catastrophic interface contact between the solid electrolyte and lithium metal. Herein, a gradient nitrogen-doping strategy by nitrogen plasma is introduced to modify the surface and subsurface of the garnet electrolyte, which not only etches the surface impurities (e.g., Li2CO3) but also generates an in situ formed Li3N-rich interphase between the solid electrolyte and lithium anode. As a result, the Li/LLZTON-3/Li cells show a low interfacial resistance (3.50 Ω cm2) with a critical current density of about 0.65 mA cm-2 at room temperature and 1.60 mA cm-2 at 60 °C, as well as a stable cycling life for over 1300 h at 0.4 mA cm-2 at room temperature. A hybrid solid-state full cell paired with a LiFePO4 cathode exhibits excellent cycling durability and rate performance at room temperature. These results demonstrate a rational strategy to enable lithium utilization in SSBs.

Tuning the Covalent Coupling Degree between the Cathode and Electrolyte for Optimized Interfacial Resistance in Solid-State Lithium Batteries

The development of promising solid-state lithium batteries has been a challenging task mainly due to the poor interfacial contact and high interfacial resistance at the electrode/solid-state electrolyte (SSE) interface. Herein, we propose a strategy for introducing a class of covalent interactions with varying covalent coupling degrees at the cathode/SSE interface. This method significantly reduces interfacial impedances by strengthening the interactions between the cathode and SSE. By adjusting the covalent coupling degree from low to high, an optimal interfacial impedance of 33 Ω cm^-2 was achieved, which is even lower than the interfacial impedance using liquid electrolytes (39 Ω cm^-2). This work offers a fresh perspective on solving the interfacial contact problem in solid-state lithium batteries.

New Network Polymer Electrolytes Based on Ionic Liquid and SiO2 Nanoparticles for Energy Storage Systems

Elementary processes of electro mass transfer in the nanocomposite polymer electrolyte system by pulse field gradient, spin echo NMR spectroscopy and the high-resolution NMR method together with electrochemical impedance spectroscopy are examined. The new nanocomposite polymer gel electrolytes consisted of polyethylene glycol diacrylate (PEGDA), salt LiBF₄ and 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF₄) and SiO₂ nanoparticles. Kinetics of the PEGDA matrix formation was studied by isothermal calorimetry. The flexible polymer-ionic liquid films were studied by IRFT spectroscopy, differential scanning calorimetry and temperature gravimetric analysis. The total conductivity in these systems was about 10^-4 S cm^-1 (-40 °C), 10^-3 S cm^-1 (25 °C) and 10^-2 S cm^-1 (100 °C). The method of quantum-chemical modeling of the interaction of SiO₂ nanoparticles with ions showed the advantage of the mixed adsorption process, in which a negatively charged surface layer is formed from Li⁺ BF₄^- ions on silicon dioxide particles and then from ions of the ionic liquid EMI⁺ BF₄^-. These electrolytes are promising for use both in lithium power sources and in supercapacitors. The paper shows preliminary tests of a lithium cell with an organic electrode based on a pentaazapentacene derivative for 110 charge-discharge cycles.

Advances in Ordered Architecture Design of Composite Solid Electrolytes for Solid-State Lithium Batteries

Solid electrolyte lithium batteries are the next generation of advanced energy devices. The incorporation of solid electrolytes can significantly improve the safety issue of lithium-ion batteries. Organic-inorganic composite solid electrolytes (CSE) are promising candidates for solid-state batteries, but their application is mainly limited by low ionic conductivity. Many studies have shown that the architecture of ordered inorganic fillers in CSE can act as fast lithium-ion transfer channels by auxiliary means, thus significantly improving the ionic conductivities. This review summarises the recent advances in CSE with different dimensional inorganic fillers. Various effective strategies for the construction of ordered structures in CSE are then presented. The review concludes with an outlook on the future development of CSE. This review aims to provide researchers with an in-depth understanding of how to achieve ordered architectures in CSE for advanced solid state lithium batteries.

Effect of the Solvate Environment of Lithium Cations on the Resistance of the Polymer Electrolyte/Electrode Interface in a Solid-State Lithium Battery

The effect of the composition of liquid electrolytes in the bulk and at the interface with the LiFePO₄ cathode on the operation of a solid-state lithium battery with a nanocomposite polymer gel electrolyte based on polyethylene glycol diacrylate and SiO₂ was studied. The self-diffusion coefficients on the 7Li, 1H, and 19F nuclei in electrolytes based on LiBF₄ and LiTFSI salts in solvents (gamma-butyrolactone, dioxolane, dimethoxyethane) were measured by nuclear magnetic resonance (NMR) with a magnetic field gradient. Four compositions of the complex electrolyte system were studied by high-resolution NMR. The experimentally obtained ¹H chemical shifts are compared with those theoretically calculated by quantum chemical modeling. This made it possible to suggest the solvate shell compositions that facilitate the rapid transfer of the Li⁺ cation at the nanocomposite electrolyte/LiFePO₄ interface and ensure the stable operation of a solid-state lithium battery.

A Multifunctional Gradient Solid Electrolyte Remarkably Improving Interface Compatibility and Ion Transport in Solid-State Lithium Battery

Solid electrolytes with both interface compatibility and efficient ion transport have been an urgent technical requirement for the practical application of solid-state lithium batteries. Herein, a multifuctional poly(1,3-dioxolane) (PDOL) electrolyte combining the gradient structure from the solid state to the gel state with the Li_6.4La₃Zr_1.4Ta_0.6O₁₂ (LLZTO) interfacial modification layer was designed, in which the "solid-to-gel" gradient structure greatly improved the electrode/electrolyte interface compatibility and ion transport, while the solid PDOL and LLZTO layers effectively improved the interface stability of the electrolyte/lithium anode and the inhibition of the lithium dendrites via their high mechanical strength and forming a stable interfacial SEI composite film. This gradient PDOL/LLZTO composite electrolyte possesses a high ionic conductivity of 2.9 × 10^-4 S/cm with a wide electrochemical window up to 4.9 V vs Li/Li⁺. Compared with the pristine PDOL electrolyte and PDOL solid electrolyte membrane coated with a layer of LLZTO, the gradient PDOL/LLZTO composite electrolyte shows better electrode/electrolyte interfacial compatibility, lower interface impedance, and smaller polarization, resulting in enhanced rate and cycle performances. The NCM622/PDOL-LLZTO/Li battery can be stably cycled 200 times at 0.3C and 25 °C. This multifunctional gradient structure design will promote the development of high-performance solid electrolytes and is expected to be widely used in solid-state lithium batteries.

Ultrahigh Elastic Polymer Electrolytes for Solid-State Lithium Batteries with Robust Interfaces

Solid polymer electrolytes (SPEs) are promising for solid-state lithium batteries, but their practical application is significantly impeded by their low ionic conductivity and poor compatibility. Here, we report an ultrahigh elastic SPE based on cross-linked polyurethane (PU), succinonitrile (SN), and lithium bistrifluoromethanesulfonimide (LiTFSI). The resulting electrolyte (PU-SN-LiTFSI) exhibits an ionic conductivity of 2.86 × 10^-4 S cm^-1, a tensile strength of 3.8 MPa, and a breaking elongation exceeding 3000% at room temperature. A solid-state lithium battery using the electrolyte exhibits a high specific capacity of 150 mAh g^-1 at 0.2C and a long cycling life of up to 700 cycles at 0.5C at room temperature, showing one of the best performances among its counterparts. The excellent performances are attributed to the fact that its ultrahigh elasticity, good ionic conductivity, tensile strength, and electrochemical stability contribute to robust electrode/electrolyte interfaces, thus greatly decreasing the charge-transfer resistance in charge/discharge processes. Our investigations provide a novel strategy to address the intrinsic interfacial issue of solid-state batteries.

Efficient Mutual-Compensating Li-Loss Strategy toward Highly Conductive Garnet Ceramics for Li-Metal Solid-State Batteries

Garnet-type Li₇La₃Zr₂O₁₂ (LLZO) is a promising solid-state electrolyte (SSE) due to its high Li⁺ conductivity and stability against lithium metal. However, wide research and application of LLZO are hampered by the difficulty in sintering highly conductive LLZO ceramics, which is mainly attributed to its poor sinterability and the hardship of controlling the Li₂O atmosphere at a high sintering temperature (∼1200 °C). Herein, an efficient mutual-compensating Li-loss (MCLL) method is proposed to effectively control the Li₂O atmosphere during the sintering process for highly conductive LLZO ceramics. The Li_6.5La₃Zr_1.5Ta_0.5O₁₂ (LLZTO) ceramic SSEs sintered by the MCLL method own high relative density (96%), high Li content (5.54%), high conductivity (7.19 × 10^-4 S cm^-1), and large critical current density (0.85 mA cm^-2), equating those sintered by a hot-pressing technique. The assembled Li-Li symmetric battery and a Li-metal solid-state battery (LMSSB) show that the as-prepared LLZTO can achieve a small interfacial resistance (17 Ω cm²) with Li metal, exhibits high electrochemical stability against Li metal, and has broad potential in the application of LMSSBs. In addition, this method can also improve the sintering efficiency, avoid the use of mother powder, and reduce raw-material cost, and thus it may promote the large-scale preparation and wide application of LLZO ceramic SSE.

Application of a Modified Porphyrin in a Polymer Electrolyte with Superior Properties for All-Solid-State Lithium Batteries

Porphyrins and their derivatives are a unique class of multifunctional and modifiable π-conjugated heterocyclic organic molecules, which have been widely applied in the fields of optoelectronic devices and catalysis. However, the application of porphyrins in polymer electrolytes for all-solid-state lithium-ion batteries (ASSLIBs) has rarely been reported. Herein, porphyrin molecules modified by polyether chains are used for composite solid-state polymer electrolytes (CSPEs) for the first time. The introduction of a modified porphyrin in an electrolyte can not only promote the electrochemical properties by constructing ordered ion channels via the intermolecular interaction between π-conjugated heterocyclic porphyrins, but also significantly improve the mechanical strength and interface contact between the electrolyte membrane and the lithium metal anode. Consequently, the all-solid-state batteries assembled by the modified porphyrin composite polymer electrolyte, LiFePO₄ cathodes, and Li anodes deliver a higher discharge capacity of 158.2 mA h g^-1 at 60 °C, 0.2 C, which remains at 153.6 mA h g^-1 after 120 cycles with an average coulombic efficiency of ∼99.60%. Furthermore, the flexible porphyrin-based composite polymer electrolyte can also enable a Li || LiCoO₂ battery to exhibit a maximum discharge capacity of 108.6 mA h g^-1 at 60 °C, 0.1 C with an active material loading of 2-3 mg cm^-2, which is unable to realize for the corresponding batteries with a pure PEO-based polymer electrolyte. This work not only broadens the application scope of porphyrins, but also proposes a novel method to fabricate CSPEs with improved electrochemical and mechanical properties, which may shed new light on the development of CSPEs for next-generation high-energy-density lithium-ion batteries.

Achieving Both High Ionic Conductivity and High Interfacial Stability with the Li2+xC1-xBxO3 Solid-State Electrolyte: Design from Theoretical Calculations

A crystalline solid electrolyte interphase Li₂CO₃ material with a large band gap shows promise toward next-generation all-solid-state lithium batteries (ASSLBs). However, the inferior ionic diffusivity restricts such structures to a real battery setup. Herein, based on density functional theory calculation and Python materials genomics, we theoretically develop the chemistry and local structural motifs to build a mixed boron-carbon framework Li_2+xC_1-xB_xO₃ (LCBO). We examine the electrochemical and chemical stabilities of LCBO-electrode interfaces by analyzing the thermodynamics of formation of interfacial phases. Interestingly, the LCBO material is automatically protected from further decomposition through the self-generated resistive interphase (Li₂CO₃ and Li₃BO₃), which gives a wide range of operating potential. LCBO shows high interfacial stability with LiCoO₂, LiMnO₂, and LiMn₂O₄. More importantly, the theoretical Li-ion migration barrier of LCBO (x = 0.375) is approximately 0.23 ± 0.02 eV through a cooperative migration mechanism. Therefore, the LCBO material combines high Li-ion diffusivity with good interfacial stability, which makes it a promising solid-state electrolyte material for ASSLBs.

Mitigating the Interfacial Degradation in Cathodes for High-Performance Oxide-Based Solid-State Lithium Batteries

Solid-state lithium batteries (SSLBs) are the promising next-generation energy storage systems because of their attractive advantages in terms of energy density and safety. However, the interfacial engineering and battery building are of huge challenges, especially for stiff oxide-based electrolytes. Herein, we construct SSLBs by a cosintering method using Li₃BO₃ as a sintering agent to bind the cathode materials LiNi_0.6Mn_0.2Co_0.2O₂ (NMC) and solid-state electrolytes Li_6.4La₃Zr_1.4Ta_0.6O₁₂. Small NMC primary particles are compared with large secondary particles to study the effects on interfacial adhesion, mechanical retention, internal resistance evolution, and electrochemical performance. Our results reveal that the interfacial resistance decreases during charging and increases during discharging, resulting in an overall increase in the interfacial resistance after one cycle. The main reason is attributed to the microcracks induced by the volumetric changes of NMC during the electrochemical process. The mechanical degradations at the interfaces accumulated upon cycling can cause capacity decay and low Coulombic efficiency. The SSLB constructed from small NMC primary particles shows regulation of particle distribution, mitigation in local volumetric change, and alleviation in mechanical degradation at the interfaces, leading to smaller resistance change and better electrochemical performance. The findings shed lights on designing SSLBs with good mechanical retention and electrochemical performance.