An interfacial compatible Ti4P8S29 polysulfide cathode with open channels for high-rate solid-state polymer sodium batteries

Abstract

Solid-state polymer sodium batteries (SPSBs) are promising candidates for achieving higher energy density and safe energy storage. However, interface issues between oxide cathode and solid-state polymer electrolyte are a great challenge for their commercial application. In contrast, soft sulfur-based materials feature better interface contact and chemical compatibility. Herein, an interfacial compatible polysulfide Ti₄P₈S₂₉ with robust Ti–S bonding and a highly active P–S unit is tailored as a high-performance cathode for SPSBs. The Ti₄P₈S₂₉ cathode possesses a three-dimensional channel structure for offering ample Na⁺ diffusion pathways. The assembled poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP)-based SPSBs deliver a discharge capacity of 136 mAh·g⁻¹ at 0.5C after 200 cycles. Furthermore, a discharge capacity of 88 mAh·g⁻¹ is retained after 600 cycles at a high rate of 2C, surpassing many cathode materials in SPSBs. A dual-site redox of Ti⁴⁺/Ti³⁺ and S⁻/S²⁻ is verified by X-ray photoelectron spectroscopy (XPS) and cyclic voltammetry (CV) tests. Interestingly, a refined locally-ordered amorphous structure is unveiled by in situ and ex situ characterizations. The as-formed electrode structure with lots of open channels and isotropic properties are more beneficial for ion diffusion on the interface of electrode and solid-state polymer electrolytes (SPEs), leading to faster Na⁺ diffusion kinetics. This work proposes a strategy of modulating open-channel to boost conversion kinetics in polysulfide cathode and opens a new pathway for designing high-performance SPSBs.

Graphical abstract