Abstract
The electrochemical lithiation and delithiation of the layered oxysulfide Sr2MnO2Cu4−δS3 has been investigated by using a combination of in situ powder X-ray diffraction and ex situ neutron powder diffraction, X-ray absorption and 7Li NMR spectroscopy, together with a range of electrochemical experiments. Sr2MnO2Cu4−δS3 consists of [Sr2MnO2] perovskite-type cationic layers alternating with highly defective antifluorite-type [Cu4−δS3] (δ ≈ 0.5) anionic layers. It undergoes a combined displacement/intercalation (CDI) mechanism on reaction with Li, where the inserted Li replaces Cu, forming Li4S3 slabs and Cu+ is reduced and extruded as metallic particles. For the initial 2–3% of the first discharge process, the vacant sites in the sulfide layer are filled by Li; Cu extrusion then accompanies further insertion of Li. Mn2.5+ is reduced to Mn2+ during the first half of the discharge. The overall charging process involves the removal of Li and re-insertion of Cu into the sulfide layers with re-oxidation of Mn2+ to Mn2.5+. However, due to the different diffusivities of Li and Cu, the processes operating on charge are quite different from those operating during the first discharge: charging to 2.75 V results in the removal of most of the Li, little reinsertion of Cu, and good capacity retention. A charge to 3.75 V is required to fully reinsert Cu, which results in significant changes to the sulfide sublattice during the following discharge and poor capacity retention. This detailed structure–property investigation will promote the design of new functional electrodes with improved device performance.
Highlights
Despite the deep penetration of Li-ion secondary batteries into the market, a need remains to continue to improve them through the optimization of currently available electrode materials and the discovery of new ones due to massive increases in market demands
The structure of the electrode material is completely reorganized, leading to metal nanoparticles embedded or surrounded in a LinX salt.[1−5] Displacement reactions are related to conversion reactions but usually some part of the framework is preserved during thelithiation processes
The charge capacity of the Sr2MnO2Cu2m−δSm+1 oxysulfides is proportional to the thickness of the sulfide layers. The cyclability of these materials has been tested in Li batteries using a voltage window of 1.1 to 2.7 V, and the results have shown that (i) the retention of charge capacity in these oxysulfides largely depends on the thickness of the Cu2S layer, the m = 3 member shows the largest capacity; (ii) better performance in cycling but lower capacity was observed for the compounds with thinner sulfide layers; and (iii) the cycling performance of antifluorite-type Cu2S under similar conditions was poor, meaning that the rigid perovskite-type [Sr2MnO2] layers in the structure seem to play a key role in providing structural stability of the framework upon Cu extrusion
Summary
Despite the deep penetration of Li-ion secondary batteries into the market, a need remains to continue to improve them through the optimization of currently available electrode materials and the discovery of new ones due to massive increases in market demands. Displacement reactions have been observed for several Cu−Sn and Cu−Sb intermetallic alloys that can function as the negative electrode materials in Li batteries[6−8] and even in materials that can act as positive electrodes.[9−11] A prominent example of the latter is Cu2.33V4O11,10 which has a structure consisting of [V4O11]n layers, and interlayer Cu cations (Cu+ and Cu2+) When it reacts with Li electrochemically, a reversible Li-driven process leading to the growth (on discharge) and disappearance (on charging) of Cu metal dendrites is observed, together with a concomitant exchange of Li for Cu in the interlayer sites.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
