Lithium-ion batteries have the advantages of high voltage, high specific energy (above 200Wh/kg), and long lifespan. They are the first choice for electric vehicle power batteries, but with the dramatic increase in the demand for power batteries, they have also boosted the related upstream raw material products. Prices, Lithium-ion battery-related raw materials, such as lithium, cobalt, nickel, etc., have all experienced significant increases in 2017. In particular, the price increase of lithium and cobalt raw materials can be described in terms of madness. As the production costs have soared and the pressure from downstream manufacturers of the industrial chain has lowered the cost, the profit margin of the power battery manufacturers has been greatly squeezed. At the same time, in the fierce market competition, the power battery manufacturers lack the pricing power. Therefore, 2018 The year will still be a very difficult year for the majority of power battery manufacturers.
Raising the performance of the power battery and reducing the production cost is the key to the future development of the battery. Recently, Cui Wei and Wei Chen of Stanford University jointly developed a MnO-based system. 2-H2The new battery, which uses an aqueous solution as the electrolyte, operates at a voltage of 1.3V. The actual specific capacity of the battery can be up to 139Wh/kg (theoretical specific capacity is about 174Wh/kg), and the cycle life can reach over 10,000 times. , And has the advantage of low cost, so it has broad application prospects in energy storage and power battery.
The positive working principle of Mn-H battery is soluble Mn 2+With solid MnO 2Between the changes, the negative is adopted H +And H 2Between the changes, the electrolyte is a high concentration of MnSO 4Different from the traditional solid-state electrodes, the positive and negative electrode reaction products are all soluble (as shown in the following formula).
The structure of the Mn-H battery is shown in the figure below. The positive electrode uses a carbon fiber felt with few holes, the separator is a glass fiber film, and the negative electrode is a structure of a carbon fiber felt loaded Pt/C composite catalyst. The electrolyte is a high concentration of MnSO. 4Solution, when charging Mn 2+It will migrate to the surface of the positive carbon fiber, and an oxidation reaction will generate a layer of MnO on the surface of the carbon fiber. 2, H+A reduction reaction occurs on the negative surface to generate H 2The discharge process is just the opposite, MnO 2When electrons are obtained, a reduction reaction occurs to generate soluble Mn 2+, return to solution to generate MnSO 4, H2An oxidation reaction occurs at the negative electrode to generate H+.
Wei Chen used the above battery structure to make a battery (shown in the figure below) and tested the electrochemical performance of the system in order to reduce the aqueous solution at high voltage. 2In the issue of positive electrode precipitation, Wei Chen set the charging voltage at 1.6V, and Wei Chen adopted 1M of MnSO. 4As the electrolyte, the first efficiency of the battery is 61%. After more than 10 cycles, the Coulomb efficiency is about 91%. Because the negative electrode Pt catalyst is more active in the acidic environment, Wei Chen added the solution. 0.05M H 2SO 4, The performance of Mn-H battery is greatly improved, the charging current is increased three times (1.6V constant voltage charging), and it takes only 85s to complete the charging. The discharge voltage platform also has a significant increase (about 50mV), and the first efficiency is also Raised to 70%, and in the following cycles, Coulomb efficiency reached about 100%.
For a power battery, the rate performance is a very important indicator. The following figure b shows the Mn-H battery in the same charging system (1.6V constant voltage charge to 1mAh/cm 2) After charging, the discharge curve at different current densities shows a discharge current density from 10 mA/cm 2, increase to 50 and 100mA/cm 2After that, the battery discharge capacity has almost no decline, which is consistent with the results of different rate cycles in the following figure c, indicating that the Mn-H battery has very excellent rate performance. More importantly, the Mn-H battery is rapidly charged In the case of the cycle, there is no decline in the capacity of 10,000 cycles.
Although the Mn-H battery has excellent rate performance and cycle performance, the utilization efficiency of the electrolyte by the carbon fiber electrode is very low, only about 36%, so the overall energy density of the battery is only 19.6Wh/kg. To solve this problem, Wei Chen uses a nanostructured carbon film as an electrode, making 4M MnSO 4The utilization efficiency of the electrolyte was improved to 74.3%, which resulted in an increase of the energy density of the battery to 139 Wh/kg and a volumetric energy ratio of 210.6 Wh/L. Wei Chen also observed further improvement in the electrolyte. 2SO 4The concentration can also effectively increase the battery's rate performance, reduce the charging time, and increase the discharge voltage level, but it is too high. 2SO 4Concentration may cause corrosion problems, which needs to be further solved from the perspective of battery structure design.
The Mn-H battery is still facing a problem - how to go from the laboratory to the application? The primary goal to solve this problem is to increase the capacity of the Mn-H battery. One measure is to increase the thickness and area of the positive carbon fiber felt. This measure can significantly increase the positive electrode load, but this will lead to an accelerated rate of capacity decline of the Mn-H battery. For example, by thickening the positive carbon fiber felt thickness by a factor of 2, although the battery capacity is increased by a factor of two, After 600 cycles, the capacity declines to 96.5% of the initial capacity. Another measure is the design of asymmetric positive and negative electrodes. From the working principle of the Mn-H battery, the negative electrode structure is mainly responsible for the catalytic effect. Does not require storage H 2Therefore, Wei Chen et al. designed the Mn-H battery into a cylindrical structure. By increasing the area of the positive electrode and reducing the area of the negative electrode (as shown in the figure below), the capacity and energy density of the Mn-H battery were significantly improved, and at the same time, the The amount of Pt/C catalyst is reduced, and the cost of the Mn-H battery is reduced. Although this design will reduce the battery's rate performance to a certain extent (reduction of negative electrode reaction area), this does not hinder the good cycle performance of the battery. As can be seen from the following figure e, after 1400 cycles, the capacity retention rate can still reach 94.2%, fully satisfying the demand for power batteries.
The Mn-H battery developed by Cui Wei and Wei Chen, etc. is essentially a hybrid battery consisting of a chemical energy storage battery cathode and a fuel cell anode, since H can not be calculated. 2The quality of the negative electrode reduces the weight of the battery and utilizes Mn 2+/Mn 4+The two electron reactions, will MnO 2The theoretical capacity has been increased to 616mAh/g, so although the voltage of the battery platform is only about 1.3V, but still get a higher specific energy. However, the current battery still has some problems, first of all, the negative electrode carbon fiber felt weight The large, poor wettability reduces the specific energy of the battery. A film made of nano-carbon material is used as the electrode, which increases the cost of the battery. In addition, since the battery generates hydrogen when the battery is charged, an airflow (such as Ar) is required. , N 2Gas, etc.) will produce H 2Bring out the battery, also need to continue to supply the battery during the discharge of H 2, this requires an additional storage outside the battery Ar (N 2) and H 2The device causes the specific energy of the battery system to decrease. There is also an implicit problem in H. 2There is a small amount of CO, CO in 2(this is the current industrial system H 2Common impurities may cause negative catalyst poisoning and affect the cycle life of the battery. These problems need to be solved in the subsequent optimization of the battery. But overall this is a very creative idea, through a good optimization Can effectively reduce the cost of the battery, for the promotion of large-scale energy storage and electric vehicles have a very important significance.
