Résumé
I ncreasing the energy density of lithium-ion batteries requires denser positive electrodes with higher voltage and/or capacity. Li-rich nickel-manganese-cobalt layered oxides (Li-rich NMCs), such as Li[Li 0.2 Ni 0.13 Mn 0.54 Co 0.13 ]O 2 , can deliver specific capacities above 270 mAh g-1 to reach 1,000 Wh kg-1 of specific energy at the material level. Despite their slightly lower crystalline density than today's Li-stoichiometric Ni-based layered oxides (Li NMCs and Li nickel-cobalt-aluminium oxides), Li-rich cathodes remain very promising for the long term, as we will need to move away from Ni-based towards Mn-based materials (Mn is inexpensive and environmentally benign) without compromising the energy density 1-3. The high capacity of Li-rich NMCs stems from cumulative anionic and cationic bulk redox processes 4-6. However, these electrodes currently fall short in other performance metrics 7 because of large voltage hysteresis 5,8 , sluggish kinetics 5,9,10 and gradual voltage fade 11. These issues are concomitant with anionic redox activity 7-the very same feature that enhances capacity. Therefore, further investigations are needed to fundamentally understand the overall anionic redox process, which constitutes not just electron removal from oxygen-based electronic states, but also the ensuing (local) struc-tural/bonding rearrangements 7. The undesirable issue of voltage hysteresis in rechargeable batteries leads to energy inefficiency, presumably dissipated as heat, consequently imposing an additional energy cost on the end users 12. Voltage hysteresis would also complicate the state of charge (SOC) and thermal management of such batteries. Li-rich NMCs show a relatively large difference between charge/discharge voltages (~400-500 mV after the first cycle and ~87% energy efficiency; Fig. 1). This gap persists over cycling, even at extremely low rates (C/300) 10 , at high temperatures (55 or 85 °C) 13 and after long relaxation periods (100 h) 10. Such behaviour therefore cannot be described by simple electro-chemical kinetics that fails to explain the observed path dependence and quasi-static hysteresis. Interestingly, and similar to Li-rich NMCs, many other newly discovered Li-and Na-based materials with reversible anionic redox also suffer from voltage hysteresis 7. These include layered Li 2 Ru 1-y Sn y O 3 9 (Fig. 1), Na 2/3 [Mn 1-y Mg y ]O 2 14 and Na 2 RuO 3 15 , as well as disordered Li 1.2 Mn 0.4 Ti 0.4 O 2 16 and Li 2 MnO 2 F 17. Only a few studies have attempted to understand the origin of voltage hysteresis in this class of cathodes. These include electro-chemical measurements in different voltage windows to identify correlated differential capacity (dQ/dV) peaks 5,8,11,18,19 , 6 Li nuclear magnetic resonance to observe path dependence in Li site occupation 20 , X-ray diffraction to claim back-and-forth (partially reversible) transition metal migration 21 , and bulk X-ray spectrosco-pies 5,22,23 to show the absence of hysteresis in the potentials at which transition metals show redox activity, unlike for the hysteretic oxygen redox process. A couple of phenomenological models assuming either an Li-driven phase change 10 or an asymmetry in transition metal migration 24 were also conceived. Despite such widespread efforts, the general thermodynamic mechanism behind voltage hys-teresis and its thermal effects remains unclear. Two questions need answering: (1) how exactly is the lost energy dissipated as heat so that it can be better managed/predicted? and (2) what is the underlying mechanism along with the corresponding thermochemical conditions that lead to hysteresis? In light of this, we adopt a different approach herein and perform isothermal calorimetry measurements during the cycling of a 'model' Li-rich layered cathode-Li 2 Ru 0.75 Sn 0.25 O 3 (LRSO) or Li[Li 0.33 Ru 0.5 Sn 0.17 ]O 2. This high-capacity (~250 mAh g-1 reversibly) material, although only suitable for niche applications without cost barriers (for example, space), shows remarkable structural and elec-trochemical similarities to the practically important Li-rich NMCs (Fig. 1) 9,25. Overall, it serves as a simplified 'model' compound for understanding the general properties of Li-rich layered electrodes. The commercialization of high-energy batteries with lithium-rich cathode materials exhibiting combined cationic/anionic redox processes awaits the elimination of certain practical bottlenecks. Among these, large voltage hysteresis remains the most obscure from a fundamental thermochemical perspective. Here, we study this issue by directly measuring, via isothermal calo-rimetry, the heat generated by Li/Li 2 Ru 0.75 Sn 0.25 O 3 (Li/LRSO) cells during various cycling conditions, with LRSO being a 'model' Li-rich layered cathode. We show how this heat thermodynamically relates to the lost electrical work that is crucial for practical applications. We further reveal that anionic redox on charging and discharging adopts different metastable paths having non-identical enthalpy potentials, such that the overall Li content no longer remains the unique reaction coordinate, unlike in fully path-reversible cationic redox. We elucidate how quasi-static voltage hysteresis is related to heat dissipated due to non-equilibrium entropy production. Overall, this study establishes the great benefits of isothermal calorimetry for enabling energy-efficient electrode materials in next-generation batteries.