Square-Scheme Electrochemistry in Battery Electrodes

Sustainable development cannot be achieved without substantial technological advancements. For instance, flexible electricity management requires smart power sourcing with advanced energy storage/conversion technologies. Remedies for abrupt power spikes/drops observed in renewable energy sources such as solar and wind require rapid load-leveling using high-power energy storage systems when they are integrated into a microgrid. Electrochemical energy storage devices efficiently convert electrical and chemical energy, which can potentially function as distributed power sources. Among these, lithium-ion batteries are a present de facto standard with their relatively high energy density and energy efficiencies that are based on topochemical intercalation chemistry, whereby guest lithium ions are (de)intercalated reversibly with simultaneous redox reactions and minimal structural changes. However, their energy density, power density, life-cycle cost, calendar life, and safety remain unsatisfactory for widespread use. When the storage capacity is maximized, as a result of which a labile deep charge/discharge state is generated, to develop batteries with high energy density, subsequent irreversible phase transformations or chemical reactions occur in many cases. The combination of the reversible electrode reactions and the subsequent irreversible phase transformations sometimes causes a charge/discharge curve characterized by a large voltage hysteresis with 100% Coulombic efficiency. Because a large voltage hysteresis significantly degrades the energy efficiency, unveiling the reaction mechanism is of primary importance in mitigating energy loss. In this Account, we comprehensively discuss the distinct and reversible charge/discharge reactions, generalized by the term "square scheme", which includes both thermodynamic and kinetic processes. The difficulties encountered in analyzing the square scheme are that both energy efficient and inefficient processes coexist and compete with each other, where the latter involves the time-dependent phenomenon. Here, we provide the theoretical models and analytical expressions for kinetic square-scheme electrodes under several electrochemical conditions, including galvanostatic charge/discharge, the galvanostatic intermittent titration technique (GITT), the potentiostatic intermittent titration technique (PITT), and constant-current/constant-voltage (CC-CV) charge/discharge. The validity of the analytical models was confirmed for two typical square-scheme electrodes: Na1-xTi0.5Co0.5O2 and Na2-xMn3O7.Na1-xTi0.5Co0.5O2, which is a sodium-ion battery cathode material, undergoes phase transitions between high-spin and low-spin states after transition-metal oxidation/reduction, while Na2-xMn3O7, which is a large-capacity oxygen-redox cathode material, exhibits O-O bond formation after oxide-ion oxidation and O-O bond cleavage after peroxide reduction, both of which trigger large voltage hysteresis. This Account emphasizes the importance of the quantitative analyses of the square scheme in which a large amount of voltage hysteresis can occur within any electrode material with a large capacity or high voltage that undergoes irreversible chemical reactions upon deep charging or discharging. Such parasitic energy-consuming transformations slowly proceed over a number of hours or days and should be carefully avoided to realize energy-efficient and stable battery systems.