Lithium metal microreference electrodes and their applications to Li-ion batteries

Li-ion batteries are nowadays widely used as power sources for a wide variety of electronic devices by virtue of their high cell voltage, high energy density and excellent cyclability. Though the performance of Li-ion batteries has been greatly improved during the last decade, it is still, to some extent, lagging behind the increasing power consumption for portable devices. To bridge this gap is a continuous task, which in turn, requires some fundamental understanding of Li-ion batteries. Prior to understanding the nature of Li-ion batteries, the features of the employed active materials must be clearly known. The physical and electrochemical properties of the active materials are briefly discussed in Chapter 2. This chapter also deals with the Lithylene technology that is used to fabricate the batteries studied in this thesis. With this technology, mechanically stable three-dimensional Li-ion batteries are made with different shapes. These preshaped batteries provide great freedom in the design of portable electronic devices. The experimental implications are explained in Chapter 3. For the purpose of in situ evaluating materials and developing mechanistic understanding of the behavior and degradation characteristics of Li-ion batteries, so-called lithium metal microreference electrodes have been developed, permitting measurement of individual electrode properties in a non-destructive way. This work has been described in Chapter 4. The substrate of the microreference electrode is a 80 µm copper wire placed in-between the positive and negative electrodes during battery construction. Lithium metal microreference electrodes were fabricated by in situ electrochemical deposition of lithium from both the positive and negative electrodes onto the copper wire. Chapter 4 also studied the influence of the depositing current density and the layer thickness on the stability of the microreference electrodes. A current density of 0.2 mA/cm2 and lithium layer thickness of 4 µm were found to be the most favorable lithium deposition condition. The microreference electrodes were sufficiently small to minimize the geometric disturbances to the battery performance while allowing in situ measurements. Due to the fact that it only contained a very small amount of lithium metal, the life-time of the microreference electrode was limited to a few weeks if subjected to continuous potential measurements. As an advantage, once the potential of the reference electrode starts to degrade after long-term measurements, the reference electrodes could be easily revived by re-depositing metallic lithium onto them. The microreference electrodes were adopted to measure in situ the potential profiles of the positive and negative electrodes during cycling. The results demonstrated that the potential of the negative electrode mainly accounts for the battery voltage change at the beginning of charge and at the end of discharge whereas the potential of the positive electrode dominated the battery voltage at the end of charge and at the beginning of discharge. The battery impedance is of practical importance as it has great impact on the rate capability of batteries. Generally, battery impedance comprises contributions from the series impedance (Rs), the positive (Rpos) and negative (Rneg) electrodes. With the help of the microreference electrodes, Rs, Rpos and Rneg were separately evaluated by means of electrochemical impedance spectroscopy (EIS) measurements-see Chapter 5. The battery impedance was studied as a function of battery state-of-charge (SoC), increasing significantly from a fully charge state (100% SoC) to a fully discharge state (0% SoC). EIS measurements revealed that Rpos was strongly dependent on SoC whereas Rneg appeared to be invariant with SoC. Based on the physical processes of Li+ ion intercalation/deintercalation, both Rpos and Rneg can be decomposed into four elements, viz, serial resistance (Rs), resistance for Li+ ions migrating through the solid electrolyte interface (SEI) layer (RSEI), charge transfer resistance (Rct) and solid-state diffusion resistance. The intercalation/deintercalation processes can be described by an equivalent circuit and the values of Rs, RSEI and Rct were extracted by fitting the measured impedance spectra to the equivalent circuit. Rs was proved to be small and independent of SoC. RSEIpos and RSEIneg attained the same values at 68% SoC and higher, strongly suggesting that the SEI layers covering the positive and negative electrodes may have similar properties. Rctneg was much smaller than Rctpos, the latter being found to be the most influencing element in battery resistance, especially for a fully discharged Li-ion battery. Noticeably, Rctpos heavily depended upon the battery SoC, declining considerably with increasing SoC. This result was confirmed by I-¿ studies that revealed an increase in Io with increasing SoC. To sum up, battery impedance originates mainly from Rpos. This effect was extremely pronounced at the battery fully discharged state. The dependence of Rctpos on the SoC was explained by the varying electrical conductivity of LixCoO2 with SoC. As charge transfer reactions take place at the electrode/electrolyte interface, electrons must transport between the current collector and the electrolyte/electrode interface, via LiCoO2 particles. The high electric resistance of LiCoO2 may severely impede the electron transport process, thereby yielding a high process resistance. On the contrary, Li0.5CoO2, corresponding to a fully charged battery, attains metal-like high electrical conductivity, which facilitates electrons movement and, consequently, the charge transfer process, resulting in a small process resistance. Based on the above hypothesis, the decrease of Rctpos with increasing SoC can be attributed, at least partly, to the change of electrical conductivity of LiCoO2 particles. It requires, however, more research work to experimentally prove this hypothesis. The results presented in Chapter 5 showed that the positive electrode was a key point if the rate and/or power capability is to be improved, as the positive electrode largely accounts for battery internal resistance. When a battery is in operation, a concentration gradient is built up in the electrolyte due to the passage of current, from which arises the diffusion overpotential. Chapter 6 discussed how to experimentally measure the concentration gradient in sealed battery systems. Subsequently, the diffusion overpotential can be determined once the concentration gradient is known. It is known that a concentration gradient of Li+ cations is established in the electrolyte when current flows through the battery. This concentration gradient was theoretically proved to be linear under galvanostatic steady-state conditions. Between any two locations along the concentration gradient an electrochemical potential difference exists from which an electric potential difference arises. Based on these findings, an equation was derived to correlate the concentration difference and the electric potential difference in the electrolyte. Thanks to the adoption of the microreference electrode concept, the electric potential difference in the electrolyte could be measured in situ even in sealed Li-ion batteries. Subsequently, the concentration difference and the concentration gradient could be calculated. Chapter 6 systematically investigated the concentration gradient over a current range of 0.1 – 1.0 C-rate. Experimental results proved the linearity of the concentration gradient and confirmed that the concentration gradient was proportional to the applied current under galvanostatic steady-state conditions. By using the known concentration gradient, the diffusion coefficient of Li+ ions in the electrolyte was determined and was found to be almost constant at different currents. Furthermore, the diffusion overpotential related to the concentration gradient was calculated to be insignificant in comparison to the overall battery overpotential over the current range studied, increasing from ~ 0.8 mV at 0.1 C to ~ 7 mV at 1.0 C. It must be pointed out that the diffusion overpotential is not proportional to the applied current and does not always remain small. By using the obtained values of the concentration gradient and the diffusion coefficient of Li+ ions, the limiting current of the batteries studied in this work was estimated to be 1.1 A, corresponding to 3.7 C-rate. When the applied current is approaching or even beyond the limiting current, the diffusion overpotential becomes extremely large, making the battery voltage reach the maximum value rapidly. Indeed, it was observed experimentally that a fully discharged Li-ion battery reached 4.2 V instantaneously when being charged at 1.17 A (3.9 C-rate). During theoretical derivation, for reasons of simplicity, the electrolyte activity was replaced by the electrolyte concentration, which can lead to experimental errors in the case of high current. This situation was discussed in Chapter 6. It was suggested not to apply this method to currents higher than 1 C-rate. In principle, this method can be applied to other battery systems with appropriate modification. Battery capacity fade is always of great concern. Chapter 7 focused on the mechanism of battery capacity degradation upon cycling. Basically, the loss of capacity stems from two causes: the loss of active material and the increase in overall battery internal impedance. The positive electrode was found to be the major factor in both aspects and contributed mostly to the loss of battery capacity. The positive electrode irreversibly lost 13% of its original capacity after 511 cycles whereas the negative electrode retained its capacity to host Li+ ions. The loss of positive active material was attributed to the comsumption of recyclable Li+ ions by the SEI layer on both the positive and negative electrodes and the formation of disordered structure in the positive electrode material. The impedance of Li-ion batteries increased upon cycling. EIS measurements showed that the impedance of the negative electrode was almost invariant during cycling. The positive electrode was responsible for the increase in the overall battery internal impedance. Similar to Chapter 5, Rctpos was again found to be crucial and contributed greatly to impedance rise. However, the cause for this increase in Rctpos remains a question to answer. It was observed with TEM that some cycled LiCoO2 particles developed extensive internal defects and microcracks. It is worthwhile to notice that some cycled LiCoO2 particles appeared to be as intact as the pristine ones, which indicates that electrochemical cycling does not occur uniformly everywhere within the positive electrode. This observation suggests that to uniformly utilize the positive electrode material may be a way to improve the cycling performance of batteries. Battery degradation has been proven to be a very complicated process. The complexity results from various factors. The capacity fade rate heavily depends on battery working conditions. The mechnism of battery degradation alters with different working conditions and battery chemistries. To well understand the mechnism of capacity fade requires more detailed investigations. Nevertheless, attention should be paid to the positive electrode for the purpose of prolonging the cycle life of Li-ion batteries.