Operando Photonic Band Gap Probe of Battery Electrode Materials

Innovative new materials are consistently emerging as electrode candidates from lithium-ion battery research, promising high energy densities and high-rate capabilities. Understanding potential structural changes, morphology evolution, degradation mechanisms and side reactions during lithiation is important for designing, optimising and assessing aspiring electrode materials. In-situ and operando analysis techniques provide a means to investigate these material properties under realistic operating conditions. Here, we demonstrate an operando spectroscopic method using photonic crystal-structured electrodes that uses the optical transmission spectrum to monitor changes to the state of charge or discharge during lithiation and the change to electrode structure, in real-time. Photonic crystals possess a signature optical response, with a photonic bandgap (or stopband) presenting as a structural colour reflection from the material. We leverage the presence of this photonic stopband, alongside its intricate relationship to the electrode structure and material phase, to correlate electrode lithiation with changes to the optical spectrum during operation. In this work, we explore the optical and electrochemical behaviour of a TiO2 anode in a lithium-ion battery, structured as an inverse opal photonic crystal. In principle, the operando technique demonstrated here is versatile and applicable to a wide range of electrochemical electrode material candidates when structured with ordered porosity akin to a photonic crystal structure

UnThree – Dimensionally ordered macroporous amorphous C/TiO2 composite electrodes for lithium-ion batteries

A facile method utilizing colloidal templating and sucrose as a carbon precursor is used to synthesize highly ordered, porous inverse opal structures as C/TiO2 nanocomposites. Material characterization shows amorphous TiO2 and a large pore size of ∼400 nm allowing for enhanced electrolyte penetration. C/TiO2 inverse opals materials as electrodes in Li-ion battery half cells demonstrate discharge and charge capacities of ∼870 mAh g−1 and 470 mAh g−1 , respectively, at a current density of 150 mA g−1 The enhanced capacities, which surpass theoretical limits for TiO2 and carbon based on intercalation reactions, are analyzed under voltammetric conditions to assess relative contributions to capacity from diffusion-limited intercalation and capacitive charge compensation reactions. The porous structure contributes to excellent capacity retention, rate performance and improved Coulombic efficiency (99.6% after 250 cycles), compared to individual carbon and TiO2 inverse opals. </p

Comparing cycling and rate response of SnO₂ macroporous anodes in lithium-ion and sodium-ion batteries

Tin oxide (SnO2) is a useful anode material due to its high capacity (1493 mAh g−1 and 1378 mAh g−1 vs Li/Li+ and vs Na/Na+, respectively) and natural abundance (tin is one of the thirty most abundant elements on Earth). Unfortunately, only moderate electrical conductivity and significant volume expansion of up to 300% for Li-ion, and as much as 520% for Na-ion can occur. Here, we use an ordered macroporous interconnected inverse opal (IO) architectures to enhance rate capability, structural integrity, and gravimetric capacity, without conductive additives and binders. Excellent capacity retention is shown during cycling vs Na/Na+ relative to Li/Li+. Cyclic voltammetry (CV) analysis, galvanostatic cycling, and differential capacity analysis extracted from rate performance testing evidence the irreversibility of the oxidation of metallic Sn to SnO2 during charge. This behavior allows for a very stable electrode during cycling at various rates. A stable voltage profile and rate performance is demonstrated for both systems. In a Na-ion half cell, the SnO2 retained >76% capacity after 100 cycles, and a similar retention after rate testing.</p

Carbon inverse opal macroporous monolithic structures as electrodes for Na-ion and K-ion batteries

Highly ordered three-dimensionally structured carbon inverse opals (IOs) produced from sucrose are stable electrodes in sodiumion and potassium-ion batteries. The walls of the ordered porous carbon structure contain short-range graphitic areas. The interconnected open-worked structure defines a conductive macroporous monolithic electrode that is easily wetted by electrolytes for Na-ion and K-ion systems. Electrochemical characterization in half-cells against Na metal electrodes reveals stable discharge capacities of 25 mAh g−1 at 35 mA g−1 and 40 mAh g−1 at 75 mA g−1 and 185 mA g−1 . In K-ion half cells, the carbon IO delivers capacities of 32 mAh g−1 at 35 mA g−1 and ∼25 mAh g−1 at 75 mA g−1 and 185 mA g−1 . The IOs demonstrate storage mechanisms involving both capacitive and diffusion-controlled processes. Comparison with non-templated carbon thin films highlights the superior capacity retention (72% for IO vs 58% for thin film) and cycling stability of the IO structure in Na-ion cells. Robust structural integrity against volume changes with larger ionic radius of potassium ions is maintained after 250 cycles in kion cells. The carbon IOs exhibit stable coulombic efficiency (>99%) in sodium-ion batteries and better coulombic efficiency during cycling compared to typical graphitic carbons.</p

Insight on the Failure Mechanism of Sn Electrodes for Sodium-Ion Batteries: Evidence of Pore Formation during Sodiation and Crack Formation during Desodiation

The development of Sn based anode materials for sodium ion batteries is mainly hindered by the limited understanding of sodiation/desodiation mechanisms inside the active material, which typically results in electrode damage. Herein, we report a post-mortem ex-situ scanning electron microscopic analysis of Sn thin film motivated by the intention to elucidate these structural mechanisms. Our results reveal for the first time that the surface of Sn electrode film becomes highly porous during sodiation with no presence of obvious cracks, a surprising result when compared to previous reports performed on Sn particles. Even more surprisingly, sequential ex-situ SEM observations demonstrate that, once the desodiation starts and reaches the second desodiation plateau (0.28 V), obvious cracks in the Sn film are instead observed along with porous islands of active material. These islands appear as aggregated particles which further split into smaller islands when the desodiation potential reaches its maximum value (2.0 V). Finally, for the first time, the experimental value of the sodium diffusion coefficient inside Sn was measured (3.9 × 10–14 cm2 s–1) using electrochemical impedance spectroscopy