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The co‐intercalation of solvent molecules along with Na+ into the crystal lattice of electrode materials is an undesired process in sodium batteries. An exception is the intercalation of ether solvated alkali ions into graphite, a fast and highly reversible process. Here, reversible co‐intercalation is shown to also be possible for other layered materials, namely titanium disulfide. Operando X‐ray diffraction and dilatometry are used to demonstrate different storage mechanisms for different electrolyte solvents. Diglyme is found to co‐intercalate into the TiS2 leading to a change in the voltage profile and an increase in the interlayer spacing (≈150%). This behavior is different compared to other solvents, which expand much less during Na storage (24% for tetrahydrofuran [THF] and for a carbonate mixture). For all solvents, specific capacities (2nd cycle) exceed 250 mAh g−1 whereas THF exhibited the best stability after 100 cycles. The solvent co‐intercalation is rationalized by density functional theory and linked to the stability of the solvation shells, which is largest for diglyme. Finally, the TiS2 electrode with diglyme electrolyte is paired with a graphite electrode to realize the first proof‐of‐concept solvent co‐intercalation battery, that is, a battery with two electrodes that both rely on reversible co‐intercalation of solvent molecules. [ABSTRACT FROM AUTHOR]

A known strategy for improving the properties of layered oxide electrodes in sodium‐ion batteries is the partial substitution of transition metals by Li. Herein, the role of Li as a defect and its impact on sodium storage in P2‐Na0.67Mn0.6Ni0.2Li0.2O2 is discussed. In tandem with electrochemical studies, the electronic and atomic structure are studied using solid‐state NMR, operando XRD, and density functional theory (DFT). For the as‐synthesized material, Li is located in comparable amounts within the sodium and the transition metal oxide (TMO) layers. Desodiation leads to a redistribution of Li ions within the crystal lattice. During charging, Li ions from the Na layer first migrate to the TMO layer before reversing their course at low Na contents. There is little change in the lattice parameters during charging/discharging, indicating stabilization of the P2 structure. This leads to a solid‐solution type storage mechanism (sloping voltage profile) and hence excellent cycle life with a capacity of 110 mAh g‐1 after 100 cycles. In contrast, the Li‐free compositions Na0.67Mn0.6Ni0.4O2 and Na0.67Mn0.8Ni0.2O2 show phase transitions and a stair‐case voltage profile. The capacity is found to originate from mainly Ni3+/Ni4+ and O2‐/O2‐δ redox processes by DFT, although a small contribution from Mn4+/Mn5+ to the capacity cannot be excluded. [ABSTRACT FROM AUTHOR]