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Potassium batteries are an emerging energy storage technology due to the large abundance of potassium, low cost, and potentially high energy density. However, it remains challenging to find suitable electrode materials with high energy density and good cycling stability due to the structural instability and kinetics issues resulting from large size K+. Herein, a durable and high-capacity K-Te battery was developed by rational design of a Te/C electrode and electrolyte salt chemistry. A well-confined Te/C cathode structure was prepared by using a commercially available activated carbon as the Te host via a melt-diffusion method. Compared to bulky Te, the confined Te/C electrode exhibited greatly improved cycling stability, specific capacity, and rate capability in K-Te batteries. Moreover, it was found that the electrolyte salts (KPF6 and KFSI) had significant impacts on the electrochemical performance of K-Te batteries. The Te/C electrode in the KPF6-based carbonate electrolyte exhibited higher specific capacity and better rate performance than the Te/C electrode in the KFSI-based one. Mechanism studies revealed that the KPF6 salt resulted in an organic species-rich solid-electrolyte interphase (SEI) on the Te/C electrode, allowing for fast electron transfer and K-ion diffusion and enhanced K-ion storage performance in K-Te batteries. In contrast, KFSI salt led to the formation of KF-rich SEI layers, which had much higher resistances for electron and K-ion transport and was less effective for the well-confined Te/C electrode. Our work finds that the Te electrode and electrolyte chemistry need to be simultaneously optimized and tailored toward K-ion storage in K-Te batteries. It is expected that the finding reported herein might be inspirable for the future development of K-chalcogen (S/Se/Te) batteries.

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K/K + (−2.93 V versus standard hydrogen potential). [7] These advantages have promoted a fast research development of PIB over the past few years, primarily focusing on anode/cathode materials and electrolyte design. [8][9][10][11][12][13][14] Currently, high-voltage (2-4 V) cathode materials are receiving intense attention, [15] such as Prussian blue analogues, [16] layered metal oxides, [9,17,18] polyanionic compounds, [19] and organic cathodes. [20,21] One big challenge of these cathodes is the relatively low capacity of less than 180 mAh g −1 , which drags down the overall energy density of a full cell. [22] Conversion-type sulfur (S) or selenium (Se) is expected to realize a high theoretical capacity of 1675 and 675 mAh g −1 , respectively. [23][24][25] However, the newly emerging K-S or K-Se batteries deliver unsatisfying capacity far away from their ideal value in practical applications, which was mainly caused by the dissolution of polysulfies/ polyselenides intermediates and poor electrical conductivity of S (5 × 10 −28 S cm −1 ) and Se (5 × 10 −3 S cm −1 ). [26,27] Poor electric conductivity results in sluggish reaction kinetics, low utilization of active materials, severe capacity decay, and unsatisfying rate performance. [28,29] Tellurium (Te), another chalcogen element, possesses an excellent electrical conductivity of 2 × 10 2 S cm −1 and a high volumetric capacity of 2621 mAh cm −3 (specific capacity of 420 mAh g −1 ), showing great potential as cathode materials for lithium/sodium-ion storage. [30,31] Sun et al. [32] pioneered the study of Te cathodes in PIB with an average potential of 1.6 V in 2020. They revealed a stepwise reaction mechanism (Te ↔ K 2 Te 3 ↔ K 5 Te 3 ) in the ether-based electrolyte (KTFSI in DEGDME). The major issues for this K-Te battery system were low discharge capacity and rapid capacity fading, probably caused by the large volume change of Te and the dissolution of polytellurides during cycling. [33,34] By increasing electrolyte concentration from 1 M to 5 m KTFSI in DEGDME, the battery delivered a higher capacity up to 409 mAh g −1 . However, severe capacity decay still occurred, which requires rational cathode structure design to confine the acute volume change of Te. Guo et al. [35] disclosed the K-ion storage mechanism of Te cathode in the carbonate-based electrolyte (1 m KFSI in EC:DEC) with K 2 Te as the final potassiation product via a two-electron reaction (2 K + Te ↔ K 2 Te). The excellent cycling stability was achieved with a specific capacity of 215.5 mAh g −1 after 100 cycles at 5C due to the superb immobilization of Te nanoparticles on the The emerging potassium-tellurium (K-Te) battery system is expected to realize fast reaction kinetics and excellent rate performance due to the exceptional electrical conductivity of Te. However, there has been a lack of fundamental knowledge about this new K-Te system, including the reaction mechanism and cathode structure design. Herein, a two-step reaction pathway from Te to K 2 Te 3 and ultimately to K 5 Te 3 is investig...