Solid-state polymer-particle hybrid electrolytes: Structure and electrochemical properties.

Solid-state electrolytes (SSEs) are challenged by complex interfacial chemistry and poor ion transport through the interfaces they form with battery electrodes. Here, we investigate a class of SSE composed of micrometer-sized lithium oxide (Li2O) particles dispersed in a polymerizable 1,3-dioxolane (DOL) liquid. Ring-opening polymerization (ROP) of the DOL by Lewis acid salts inside a battery cell produces polymer-inorganic hybrid electrolytes with gradient properties on both the particle and battery cell length scales. These electrolytes sustain stable charge-discharge behavior in Li||NCM811 and anode-free Cu||NCM811 electrochemical cells. On the particle length scale, Li2O retards ROP, facilitating efficient ion transport in a fluid-like region near the particle surface. On battery cell length scales, gravity-assisted settling creates physical and electrochemical gradients in the hybrid electrolytes. By means of electrochemical and spectroscopic analyses, we find that Li2O particles participate in a reversible redox reaction that increases the effective CE in anode-free cells to values approaching 100%, enhancing battery cycle life.

Solid-Adsorbed Polymer-Electrolyte Interphases for Stabilizing Metal Anodes in Aqueous Zn and Non-Aqueous Li Batteries.

Polymers are known to adsorb spontaneously from liquid solutions in contact with high-energy substrates to form configurationally complex, but robust phases that often exhibit higher durability than might be expected from the individual physical bonds formed with the substrate. Rational control of the physical, chemical, and transport properties of such interphases has emerged as a fundamental opportunity for scientific and technological advances in energy storage technology but requires in-depth understanding of the conformation states and electrochemical effect of the adsorbed polymers. Here, we analyze the interfacial adsorption of oligomeric polyethylene glycol (PEG) chains of moderate sizes dissolved in protic and aprotic liquid electrolytes and find that there is an optimum polymer molecular weight of approximately 400 Da at which the highest columbic efficiency is achieved for both Zn and Li deposition. These findings point to a simple, versatile approach for extending the lifetime of batteries.

Design principles for heterointerfacial alloying kinetics at metallic anodes in rechargeable batteries.

How surface chemistry influences reactions occurring thereupon has been a long-standing question of broad scientific and technological interest. Here, we consider the relation between the surface chemistry at interfaces and the reversibility of electrochemical transformations at rechargeable battery electrodes. Using Zn as a model system, we report that a moderate strength of chemical interaction between the deposit and the substrate-neither too weak nor too strong-enables highest reversibility and stability of the plating/stripping redox processes. Focused ion beam and electron microscopy were used to directly probe the morphology, chemistry, and crystallography of heterointerfaces of distinct natures. Analogous to the empirical Sabatier principle for chemical heterogeneous catalysis, our findings arise from competing interfacial processes. Using full batteries with stringent negative electrode-to-positive electrode capacity (N:P) ratios, we show that such knowledge provides a powerful tool for designing key materials in highly reversible battery systems based on Earth-abundant, low-cost metals such as Zn and Na.

Textured Electrodes: Manipulating Built-In Crystallographic Heterogeneity of Metal Electrodes via Severe Plastic Deformation.

Control of crystallography of metal electrodeposit films has recently emerged as a key to achieving long operating lifetimes in next-generation batteries. It is reported that the large crystallographic heterogeneity, e.g., broad orientational distribution, that appears characteristic of commercial metal foils, results in rough morphology upon plating/stripping. On this basis, an accumulative roll bonding (ARB) methodology-a severe plastic deformation process-is developed. Zn metal is used as a first example to interrogate the concept. It is demonstrated that the ARB process is highly effective in achieving uniform crystallographic control on macroscopic materials. After the ARB process, the Zn grains exhibit a strong (002) texture (i.e., [002]Zn //ND). The texture transitions from a classical bipolar pattern to a nonclassical unipolar pattern under large nominal strain eliminate the orientational heterogeneity of the foil. The strongly (002)-textured Zn remarkably improves the plating/stripping performance by nearly two orders of magnitude under practical conditions. The performance improvements are readily scaled to achieve pouch-type full batteries that deliver exceptional reversibility. The ARB process can, in principle, be applied to any metal chemistry to achieve similar crystallographic uniformity, provided the appropriate temperature and accumulated strains are employed. This concept is evaluated using commercial Li and Na foils, which, unlike Zn (HCP), are BCC crystals. The simple process for creating strong textures in both hexagonal and cubic metals and illustrating the critical role such built-in crystallography plays underscores opportunities for developing highly reversible thin metal anodes (e.g., hexagonal Zn, Mg, and cubic Li, Na, Ca, Al).

Engineering Multiscale Coupled Electron/Ion Transport in Battery Electrodes.

Coupled electron/ion transport is a defining characteristic of electrochemical processes, for example, battery charge/discharge. Analytical models that represent the complex transport and electrochemical processes in an electrode in terms of equivalent electrical circuits provide a simple, but successful framework for understanding the kinetics of these coupled transport phenomena. The premise of this review is that the nature of the time-dependent phase transitions in dynamic electrochemical environments serves as an important design parameter, orthogonal to the intrinsic mixed conducting properties of the active materials in battery electrodes. A growing body of literature suggests that such phase transitions can produce divergent extrinsic resistances in a circuit model (e.g., Re, describing electron transport from an active electrode material to the current collector of an electrode, and/or Rion, describing ion transport from a bulk electrolyte to the active material surface). It is found that extrinsic resistances of this type play a determinant role in both the electrochemical performance and long-term stability of most battery electrodes. Additionally, successful suppression of the tendency of extrinsic resistances to accumulate over time is a requirement for practical rechargeable batteries and an important target for rational design. We highlight the need for battery electrode and cell designs, which explicitly address the specific nature of the structural phase transition in active materials, and for advanced fabrication techniques that enable precise manipulations of matter at multiple length scales: (i) meso-to-macroscopic conductive frameworks that provide contiguous electronic/ion pathways; (ii) nanoscale uniform interphases formed on active materials; and (iii) molecular-level structures that promote fast electron and/or ion conduction and mechanical resilience.

Local electronic structure variation resulting in Li 'filament' formation within solid electrolytes.

Solid electrolytes hold great promise for enabling the use of Li metal anodes. The main problem is that during cycling, Li can infiltrate along grain boundaries and cause short circuits, resulting in potentially catastrophic battery failure. At present, this phenomenon is not well understood. Here, through electron microscopy measurements on a representative system, Li7La3Zr2O12, we discover that Li infiltration in solid oxide electrolytes is strongly associated with local electronic band structure. About half of the Li7La3Zr2O12 grain boundaries were found to have a reduced bandgap, around 1-3 eV, making them potential channels for leakage current. Instead of combining with electrons at the cathode, Li+ ions are hence prematurely reduced by electrons at grain boundaries, forming local Li filaments. The eventual interconnection of these filaments results in a short circuit. Our discovery reveals that the grain-boundary electronic conductivity must be a primary concern for optimization in future solid-state battery design.