Electrochemical energy storage technologies represented by lithium-ion batteries and electrochemical capacitors have played key roles in the modern smart-grid. However, the rapid development of the terminal markets, including electric vehicles and portable intelligent electronic devices, requires significant breakthroughs in respect to energy density, power density, cost, life cycle and safety, etc This means an urgent need to go beyond the current energy-storage technologies of lithium-insertion and electrical double-layer chemistry. Because of the high energies involved, the alloy-type and conversion-type chemistries have caused widespread concern in recent years [1–5]. However, they often suffer more significant problems, including poor kinetics, large volume changes and the presence of soluble intermediates. To solve these issues, the major focus has been on material engineering by nanotechnology. Through morphology control and nanostructuring, many novel active materials have showed promising electrochemical performance in a laboratory cell [6–10]. However, nanotechnology that is cost-effective and can be used on an industrial scale is still lacking. On the other hand, in many studies, lithium metal was directly used as a counter electrode, which may form lithium dendries and cause significant safety issues, such as short-circuit, burning and explosion. In this perspective, we will show that electrochemical modification can be a more efficient and beneficial way of developing high-performance energy-storage devices than material engineering. A schematic of electrochemical modification is shown in figure 1. The modified materials are then used as the cathode and anode in an electrochemical system where they are matched with a specific electrolyte and an auxiliary electrode. During modification, two individual circuits of anode//auxiliary electrode (V1) and cathode//auxiliary electrode (V2) are connected. Rational electrochemical-reaction techniques, i.e. galvanostatic discharge/charge, constant voltage discharge/charge, pulse charge, short circuit, etc, are conducted to produce changes in the electrode properties, such as surface chemistry, charge density and crystal structure. The auxiliary electrode plays the same role as a counter electrode, which should have a larger area than the modified electrode. As a result, the polarization of auxiliary electrode can be ingnored during modification, and thus the modification can be conducted effectively. Moreover, the reaction products of auxiliary electrode should not affect the reaction of modified electrode. Common auxiliary electrodes includes the copper, platinum, lithium and so on. After modification, an advanced energy-storage device that combines the optimized cathode and anode (V3) is obtained which gives an excellent electrochemical performance. Electrochemical modification has the following advantages. First, it can be performed after electrode preparation or even device assembly, which is compatible with the commercial fabrication. Second, the properties of the active materials can be quantitatively changed by controlling the parameters, such as potential, current density and capacity. At present, the electrochemical modification strategies have been widely used in the field of energy-storage and can be divided into three types: electrochemical coating, capacitive-type charge injection and battery-type charge injection. © 2020 The Author(s). Published by IOP Publishing Ltd
