Surface Electrolyte Interface Research Articles

Trimethyl borate and triphenyl borate as electrolyte additives for LiFePO4 cathode with enhanced high temperature performance

Alkyl- and phenyl-substituted borate anion receptors, triphenyl borate (TPB) and trimethyl borate (TMB), are investigated as electrolyte additives in lithium ion batteries (LiFePO4 cathode, 1 M LiPF6 EC-DMC 1:1 electrolyte) at 25 and 60 °C. TPB shows formation of a thick surface/electrolyte interface (SEI) which hampers ion flow and does not protect the electrode from degradation. Although useful at 25 °C, TPB at 60 °C results in only 7% capacity after 100 cycles, whereas 0.1 M TMB results in 53% capacity. TMB performance improves with increasing concentration. TMB also suppresses thermal decomposition of the electrolyte and formation of the SEI film.

The stability of the SEI layer, surface composition and the oxidation state of transition metals at the electrolyte–cathode interface impacted by the electrochemical cycling: X-ray photoelectron spectroscopy investigation

The stability of the valence state of the 3d transition metal ions and the stoichiometry of LiMO(2) (M = Co, Ni, Mn) layered oxides at the surface-electrolyte interface plays a crucial role in energy storage applications. The surface oxidation/reduction of the cations caused by the contact of the solids to air or to the electrolyte results in the blocking of the Li-transport through the interface that leads to the fast batteries deterioration. The influence of the end-of-charge voltage on the chemical composition and the oxidation state of 3d transition metal ions, as well as the stability of the solid-electrolyte interface formed during the electrochemical Li-deintercalation/intercalation of the LiCoO(2) and Li(Ni,Mn,Co)O(2), have been investigated by X-ray photoelectron spectroscopy. While the chemical composition of the solid-electrolyte interface is similar for both layered oxide surfaces, the electrochemical cycling to some critical voltage values leads to the disappearance of the interface. By the analysis of the shape of the 2p and 3s photoelectron emissions we show that the formation of the solid-electrolyte interface layer correlates with the partial reduction of the trivalent Co ions at the electrolyte-LiCoO(2) interface and the amount of the Co(2+) ions is increased as the solid-electrolyte interface vanishes. In contrast, the Mn(4+), Co(3+) and Ni(2+) ions of the Li(Ni,Mn,Co)O(2) are stable at the interface under the electrochemical cycling to higher end-of-charge voltage. A correlation between deterioration of the LiCoO(2) and Li(Ni,Mn,Co)O(2) batteries and the change of electronic structure at the surface/interface after the electrochemical cycling has been found. The dissolution of the solid-electrolyte interface layer might be the reason for the fast deterioration of the Li-ion batteries.

Kinetic analysis of the Li + ion intercalation behavior of solution derived nano-crystalline lithium manganate thin films

In the present work, we have optimized the process parameters to synthesize nano-crystalline lithium manganate (LMO) thin films with dense surface morphology and uniform grain size distribution by a solution growth technique. These films were characterized in terms of their phase formation, surface morphology, charge–discharge capacity, rate capability, and cycleability. The films exhibited excellent discharge capacity (close to the theoretical capacity of LMO) and rate capability in a wide range of discharge current densities. However, the rapid capacity fading was observed within the first few initial charge–discharge cycles and with progressive cycling marginal capacity fading was observed. To understand these phenomena, we have investigated the kinetics of Li ion diffusion in these thin film electrodes by cyclic voltammetry (CV) as well as potential step chronoamperometry (PSCA) measurements. In case of the cycled thin film electrode, the Li ion diffusion coefficient was reduced to almost one order of magnitude as compared to its virgin counterpart. During the charge–discharge cycling of these thin film electrodes in a liquid electrolyte, a surface electrolyte interface (SEI) layer was formed on the thin film electrode. It has been argued that the formation of such layer would reduce the Li ion diffusion coefficient and led to the observed capacity fading.

Electrolytic oxidation of anthracite

An anthracite slurry can be oxidized only with difficulty by electrolytic methods in which aqueous electrolytes are used if the slurry is confined to the region of the anode by a porous pot or diaphragm. However, it can be easily oxidized if the anthracite itself is used as the anode. No porous pot or diaphragm is needed. Oxidative consumption of the coal to alkali-soluble compounds is found to proceed preferentially at the edges of the aromatic planes. An oxidation model is proposed in which the chief oxidants are molecular and radical species formed by the electrolytic decomposition of water at the coal surface-electrolyte interface. The oxidation reactions proposed account for the opening of the aromatic rings and the subsequent formation of carboxylic acids. The model also explains the observed anisotropic oxidation and the need for the porous pot or diaphragm used in previous studies of the oxidation of coal slurries.