Enabling long cycle life lithium-ion capacitors through electrode-electrolyte interface optimization

Electrode-electrolyte interfaces in electrochemical cells represent critical boundaries between the electrode and electrolyte where most charge transfer or storage processes are initiated. Accordingly, the interface properties can influence the performance of electrochemical cells, affecting the capacity, energy and power density, electrode stability and cycle life. This is particularly significant in lithium-ion capacitors (LiC), a hybrid energy storage consisting of high surface area capacitive cathodes such as activated carbon (AC) from Supercapacitors (SC) and intercalation/alloying anodes such as graphite or hard carbon (HC) from Lithium-ion batteries (LiB) while using similar LiB electrolytes with large operating potential windows. However, the AC widely adopted as LiC cathode often possesses surface oxygenated functionalities and numerous defects from the extensive activation treatment to impact the high surface area. As a result, the AC electrode is enriched in active sites for accelerated electrolyte decomposition, which can cause rapid capacity fade and failure of such cells. Furthermore, the sensitive LiPF6 salt widely adopted in LiB electrolyte formulations is prone to hydrolysis, which may inadvertently proceed upon contact with confined moisture in the AC pores. Degradation by-products, such as HF and PF5, are formed, which may catalyse other component degradation and are detrimental to interfacial stability. Consequentially, the cycle life of the LiC is affected by these processes occurring on the AC and remains inferior compared to SC. In the first part of this study, the capacity fade mechanism of the AC cathode in a LiC cell was investigated by a combination of electrochemical and post-mortem material characterisation techniques. First, the stability was probed using sustained floating voltage holds and intermittent electrochemical spectroscopy to examine the interfacial transformations during cycling. Then, the AC electrode surface was further examined ex-situ, utilising a variety of post-mortem characterisations after different durations of cycling to reveal changes in the surface area, defect level, and surface species with the capacity fade. Finally, the results showed that the capacity fade was caused by the synergistic effect of electrolyte degradation and active material interface transformation by the deposited degradation products. The second part examined the effect of electrolyte dielectric on the performance of the AC in symmetric cell configuration and in half cells with lithium counter electrodes. Electrolyte solutions with varying dielectrics were prepared and tested in the mentioned cellular configuration. High electrolyte dielectric was revealed to improve cycle life by delaying the PF6- anion degradation at the AC electrode surface. Furthermore, the increased electrolyte dielectric enhanced the oxidative stability of the PF6- anion through a sufficiently strengthened solvation shell. These properties enhanced the cycle life and decreased features associated with electrolyte degradation in the high-dielectric electrolyte. The third part investigated the effect of electrolyte dielectric on the interfacial properties of the AC cathode and Hard carbon (HC) anode in a LiC full cell. Post-mortem characterisations and electrochemical impedance analysis were used to probe the AC and HC electrode interfaces. High electrolyte dielectric was revealed to be particularly beneficial for the AC cathode, with a three-fold decrease in interfacial impedance observed as the electrolyte dielectric was increased. Overall, increasing the electrolyte dielectric presents a method for extending the cycle life of AC LiC

Lithium-Ion Capacitors: A Review of Strategies toward Enhancing the Performance of the Activated Carbon Cathode

Lithium-ion capacitors (LiC) are promising hybrid devices bridging the gap between batteries and supercapacitors by offering simultaneous high specific power and specific energy. However, an indispensable critical component in LiC is the capacitive cathode for high power. Activated carbon (AC) is typically the cathode material due to its low cost, abundant raw material for production, sustainability, easily tunable properties, and scalability. However, compared to conventional battery-type cathodes, the low capacity of AC remains a limiting factor for improving the specific energy of LiC to match the battery counterparts. This review discusses recent approaches for achieving high-performance LiC, focusing on the AC cathode. The strategies are discussed with respect to active material property modifications, electrodes, electrolytes, and cell design techniques which have improved the AC’s capacity/capacitance, operating potential window, and electrochemical stability. Potential strategies and pathways for improved performance of the AC are pinpointed.publishedVersio

Revealing mechanisms of activated carbon capacity fade in lithium-ion capacitors

The capacity fade mechanism of activated carbon (AC) electrode in Li-ion electrolyte was studied via electrochemical impedance spectroscopy (EIS) and post-mortem electrode characterizations at different stages of electrochemical cycling. Electrochemical cycling was conducted in half cells incorporating the AC working electrode, Li metal counter electrode, and 1 M LiPF6 in EC:DMC (1:1) electrolyte. Three phases were identified during the ageing process that corresponded with transformation of the passivation layer at the electrode surface and charge transfer impedance derived from the EIS analysis. Surface area and morphology analysis showed that the AC surface was progressively transformed by degradation products that reduced the available surface area and accessibility of electrolyte moieties into the pores. X-ray photoelectron spectroscopy suggested that the degradation products are from the LiPF6 salt decomposition and carbonate solvent decomposition, while Raman analysis demonstrated increased defects in the electrode as cycling progressed. The capacity fade was therefore caused by the synergistic effect of electrolyte degradation and active material transformation.publishedVersio

Sulfur‐Decorated Ni−N−C Catalyst for Electrocatalytic CO2 Reduction with Near 100 % CO Selectivity

Developing highly efficient electrocatalysts for electrochemical CO2 reduction (ECR) to value-added products is important for CO2 conversion and utilization technologies. In this work, a sulfur-doped Ni−N−C catalyst is fabricated through a facile ion-adsorption and pyrolysis treatment. The resulting Ni−NS−C catalyst exhibits higher activity in ECR to CO than S-free Ni−N−C, yielding a current density of 20.5 mA cm−2 under −0.80 V versus a reversible hydrogen electrode (vs. RHE) and a maximum CO faradaic efficiency of nearly 100 %. It also displays excellent stability with negligible activity decay after electrocatalysis for 19 h. A combination of experimental investigations and DFT calculations demonstrates that the high activity and selectivity of ECR to CO is due to a synergistic effect of the S and Ni−NX moieties. This work provides insights for the design and synthesis of nonmetal atom-decorated M−N−C-based ECR electrocatalysts.publishedVersio

Enabling Increased Delithiation Rates in Silicon-Based Anodes through Alloying with Phosphorus

The capability of battery materials to deliver not only high lithium storage capacity, but also the ability to operate at high charge/discharge rates is an essential property for development of new batteries. In the present work, the influence on the charge/discharge rate behaviour of substoichiometric concentrations of phosphorus (P) in silicon (Si) nanoparticles was studied. The results revealed an increase in rate capability as a function of the P concentration between 0 and 5.2 at %, particularly during delithiation. The stoichiometry of the nanoparticles was found to strongly affect the formation of the Li3.5Si phase during lithiation. Cyclic stability experiments demonstrated an initial increase in capacity for the SiPx materials. Galvanostatic intermittent titration technique and electrochemical impedance spectroscopy demonstrated the increased lithium diffusivity with inclusion of P. Density functional theory and ab initio molecular dynamics were deployed to provide a rationale for the electrochemical behaviour of SiPx.publishedVersio

Enabling Increased Delithiation Rates in Silicon-Based Anodes through Alloying with Phosphorus

The capability of battery materials to deliver not only high lithium storage capacity, but also the ability to operate at high charge/discharge rates is an essential property for development of new batteries. In the present work, the influence on the charge/discharge rate behaviour of substoichiometric concentrations of phosphorus (P) in silicon (Si) nanoparticles was studied. The results revealed an increase in rate capability as a function of the P concentration between 0 and 5.2 at %, particularly during delithiation. The stoichiometry of the nanoparticles was found to strongly affect the formation of the Li3.5Si phase during lithiation. Cyclic stability experiments demonstrated an initial increase in capacity for the SiPx materials. Galvanostatic intermittent titration technique and electrochemical impedance spectroscopy demonstrated the increased lithium diffusivity with inclusion of P. Density functional theory and ab initio molecular dynamics were deployed to provide a rationale for the electrochemical behaviour of SiPx.publishedVersio

On the Viability of Lithium Bis(fluorosulfonyl)imide as Electrolyte Salt for Use in Lithium-Ion Capacitors

Lithium-ion capacitors (LICs) represent promising high-power energy storage devices, most commonly composed of a lithium-ion intercalation anode (e. g., graphite or hard carbon), a supercapacitor activated carbon (AC) cathode, and an electrolyte with 1 M LiPF6 in carbonate solvents. LiPF6 is susceptible to hydrolysis, forming HF, which leads to challenges for disassembly and recycling, risks during hazardous events, and extensive energy consumption during production. Here, we report on the feasibility of replacing LiPF6 with the non-hydrolysing salt LiFSI for use with AC electrodes. Based on voltage hold measurements in a half-cell setup, good long-term stability is achieved with an upper cut-off voltage of 3.95 V vs. Li/Li+, potentially enabling cell voltages of ~3.8 V when combined with graphite or silicon-based anodes (operating at ~0.1 V vs. Li/Li+) in LIC full cells. The lower cut-off voltage was determined to be 2.15 V vs. Li/Li+. The systematic comparison of CV, leakage current analysis and capacity retention upon voltage hold highlights the importance of the latter method to provide a realistic assessment of the electrochemical stability window (ESW) of LiFSI on a commercial AC electrode. The morphological and surface-chemical post-mortem analysis of AC electrodes used with LiFSI revealed that the oxidation of the FSI anion, as evidenced by the presence of new S 2p and N 1s features in the XPS spectra, and an increasing number of oxygenated species on the AC were the main processes causing capacity fade at positive polarization.publishedVersio