Mechanism of the interfacial reaction between cation-deficient La_0.56Li_0.33TiO₃ and metallic lithium at room temperature

Abstract

We used x-ray diffractometry (XRD), x-ray photoelectron spectrometry (XPS), and secondary-ion mass spectrometry (SIMS) to investigate the mechanism of the interfacial room-temperature (RT) chemical reaction between cation-deficient La0.56Li0.33TiO3 solid electrolytes and metallic lithium anodes in all-solid-state lithium batteries. A stoichiometric mixture of La2O3, Li2CO3, and TiO2 powders was calcined at 1250 °C for 8 h to obtain a single perovskite structure of La0.56Li0.33TiO3. When this La0.56Li0.33TiO3 sample and lithium were placed in contact at room temperature for 24 h, the phase of the La0.56Li0.33TiO3 remained unchanged. The XPS results indicate that 12% of the tetravalent Ti4+ ions were converted into trivalent Ti3+ ions. The valence conversion and degree of conversion were limited by the structural rigidity of the host crystal. Our SIMS analysis suggests the existence of a local electric field near the contact surface and indicates that the 6Li+ isotope ions were inserted into the specimen through the effect of this field. The change in the electrical properties of La0.56Li0.33TiO3 supports this mechanism for the interfacial reaction. The ionic conductivities of the grain and total grain boundary decreased and increased, respectively, after the insertion of Li+, and the total electronic conductivity increased as a result of the presence of intervalence electron hopping between mixed Ti3+/Ti4+ states. The mechanism of the lithium-activated RT interfacial reaction is associated with the reduction of Ti4+ transition metal ions from tetravalent to trivalent states and the local-electric-field-induced Li+ insertion into La3+/Li+-site vacancies of La0.56Li0.33TiO3.

Electrochemical and mechanical stability of LixLa0.557TiO3‐δ perovskite electrolyte at various voltages

“…However, a new signal appears at the Ti 3+ peak for LLTO‐0.2 V. The molar ratios of Ti 4+ and Ti 3+ in LLTO‐0.2 V are ~91% and ~9%, respectively. The Ti 4+ is partly reduced by lithiation, which is consistent well with literatures . The La 3+ signals are same for all the samples, indicating that La 3+ is stable against charging (or discharging), and it is reported that La 3+ is stable even when directly in contact with Li metal …”

“…Whereas, one side of the solid electrolyte is in contact with the cathode and the other side is in contact with the anode in a battery, suggesting that the solid electrolyte is working under a voltage gradient. The formation of Ti 3+ species has been observed at the interface between Li 0.33 La 0.56 TiO 3 electrolytes and lithium metal, and even titanium metal has formed for Li 0.35 La 0.55 TiO 3 after long‐term contacting with lithium metal . As a result, the phase stability, electrochemical stability, and mechanical stability are affected by the voltage gradient.…”

Improvement in electronic conductivity of perovskite electrolyte by variable‐valence element doping

“…Alternatively, the doping with variable‐valence elements is an effective approach to improve the electronic conductivity, which could provide donor or acceptor levels. The electronic conductions in oxides doped with variable‐valence elements are typically classified as hopping transport . All doped elements can effectively improve the electronic conductivity (Figure 4C).…”

“…Ti 3+ ions largely increase the electronic conductivity and the reduction of Ti 4+ to Ti 3+ could be achieved by Li insertion or O removal. [40][41][42] The reaction of Ti 4+ /Ti 3+ by Li insertion occurs only when the voltage is below 2 V vs Li/Li + . 35 However, the voltage of the cathode coating is usually above 2 V versus Li/Li + during the operation.…”

Amorphous Li4Ti5O12 Thin Film with Enhanced Lithium Storage Capability and Reversibility for Lithium‐Ion Batteries

“…By subtracting the normalized Ti 2p spectra of A-LTO from C-LTO, the two extra peaks at approximately 463.5 and 458.0 eV were attributed to Ti 3 + 2p1/2 and Ti 3 + 2p3/2 peaks; one reason for this phenomenon is the reduction of tetravalent Ti 4 + to trivalent Ti 3 + ions, resulting in the generation of high Li + concentrations. [27][28][29] The O 1s core level XPS spectra of A-LTO and C-LTO were also recorded and compared in Figure 4 b. Unlike the single O 1s peak at approximately 530.4 eV for C-LTO, the A-LTO exhibits a broader O 1s peak that can be resolved into two peaks at 529.9 and 532.0 eV.…”