HOW DOES A BATTERY WORK?
Batteries are small electronic devices that power everything from smartphones to electric cars. Batteries release electricity by converting stored chemical energy into electrical energy. Inside the battery there are two electrode, the anode and the cathode, which are separated by a liquid called the electrolyte. There are two important particles that drive a battery, ions and electrons. The ions, which have a positive charge, can swim through the electrolyte liquid, but the electrons, which have a negative charge, cannot swim and must run through cables when moving between the two electrodes.

As illustrated in Figure 1, there are two ways to "operate" a battery: charging (storing energy) and discharging (using stored energy). When you charge the battery in your phone, you provide energy to the battery that pumps the electrons into the anode. There, the electrons can connect to their ion partners. When you use the battery in your phone, the electrons want to rush to the cathode. We call this flow of electrons a current. The current is guided by electronic circuits to power devices like your phone. Eventually, the current runs out and the battery is discharged. Certain batteries can be recharged by forcing electrons back into the anode. The lithium battery is the best and most used rechargeable battery.

BUILDING BATTERIES FOR THE FUTURE
As our electronic devices become more advanced, their appetite for power increases. Battery researchers around the world are working hard to create new and improved batteries. There are many parts of a battery that can be improved, but the most important goal is to increase the amount of energy that can be stored in it.

To increase the energy store in batteries, we need to find better electrodes. One very promising electrode uses the element lithium. Lithium is the lightest metal on earth. It is also very reactive, which means it is the metal that most willingly releases electrons. This makes lithium perfect for portable batteries. Lithium was first used in a battery invented by Professor Whittingham in the 1970s, Sadly, lithium is very difficult to control and can cause the battery to burn. Professors Yoshino and Goodenough changed the electrodes in the first lithium battery to produce the safe version we now use in our daily lives. These important achievements awarded the three professors the Nobel Prize in Chemistry in 2019.

THE LITHIUM PROBLEM
Scientists across the world have never let go of the dream of using lithium to make better batteries. For nearly 40 years, battery scientists have tried all possible ways to make lithium safer. The problem is very difficult, and we have not yet found a solution. So, why is lithium unsafe? It likes to grow into wires inside the battery!

We recently investigated this lithium problem to fins a way of controlling how lithium grows. We started by investigating how lithium wires are formed on the anode. Figure 2 shows the growth of lithium wires. When a positive lithium ion meets a negative electron, they react and form a neutral (no electrical charge) lithium atom on the electrode surface. More and more ions meet their electron partners and form atoms on the surface. Normally, this process happens very quickly and the newly formed lithium atoms have very little time to arrange themselves in even layers. Instead, the atoms build upon each other and grow as wires into the electrolyte. The lithium ions now have a shorter path to swim to the growing lithium wire. The wires will attract nearby ions and grow faster than the rest of the electrode surface. Eventually, the lithium wires grow so long that they connect  to the opposite electrode side. Electrons can now move down the wire instead of taking the detour around the battery (through the electronic circuits of the device) to reach the lithium electrode. The shorter path is called a short-circuit and it will make the battery unsafe.

How can this dangerous problem be prevented? We studied this by looking at lithium electrodes inside a battery. The batter was first charged to grow new lithium atoms on the anode surface. Then we opened the battery and removed the lithium electrode. We looked at the electrode surface with a special electron microscope that can see objects as small as a few atoms. Next, we varied the ingredients in the battery and the way electricity was fed into the battery during charging. We found three main factors that control how lithium grows (Figure 3).

DECREASING THE FLOW OF ELECTRONS
The first factor that affects the growth of lithium wires is current density, which is the flow of electrons through the battery. High current density means many electrons moving between the electrodes at the same time. When few electrons move, the current is low. The current is like traffic in a city. Without any speed limits, the electrons will rush to the electrode and grab any partner ion that comes close to the electrode surface. With its high current, the electron-ion pairs will land wherever they can. If we instead add speed limits in the battery, the current density will be low. This means fewer electrons on the electrode surface. The electrons will now have time to move around and find a comfy spot where they can grab an ion and form a lithium atom. Low current density gives more time for the lithium atoms to be placed in an ordered patter. This means we can help lithium to grow into layers instead of wires by adding speed limits for the electrons. Layered lithium makes the battery safe to use, because no wires can grow between anode and cathode to cause a dangerous short circuit.

DECREASING LITHIUM IONS IN THE ELECTROLYTE
The second factor affecting the growth of lithium wires is the ion concentration in the electrolyte. When the electrolyte is full of ions (high concentration), they will swim around looking for electrons to react with. Like we saw with the electrons, this behaviour causes atoms to quickly grow on top of each other into wires. If we instead lower the ion concentration, then fewer ions will swim around, which gives them time to find a good spot on the electrode surface. It is important to understand that, when lowering the ion concentration, we do not change the stored energy in the battery. The energy in the battery depends on how much lithium we have in the electrodes, not in the electrolyte, which means that we can change the ion concentration to control how many ions move at the same time, without making the battery worse.

INCREASING THE NUMBER OF "PARKING SPOTS"
The third factor affecting the growth of lithium wires is the nuclei density on the electrode surface. Nuclei are small islands of atoms that act as spots for lithium growth to start. Nuclei are like parking spots for atoms. Low nuclei density means fewer parking spots, which forces the lithium atoms to pile on top of each other. If we increase the nuclei density and offer more parking spots, then the lithium atoms can all find a spot and neatly arrange themselves. High nuclei density can help the atoms to grow into layers, not wires.

OUR TEST BATTERY
In our research, we focused on changing the ion concentration and nuclei density to see if we could decrease the formation of lithium wires. We built two similar batteries. Battery 1 had a high ion concentration and low nuclei density. Battery 2 had a low ion concentration and a high nuclei density. The two batteries were then charged and discharged. Afterwards, we took the batteries apart at the lithium anode surface. In Figure 3, we show the photographs taken with the electron microscope so you can see what the surfaces of these batteries looked like. The electrode from battery 1 was covered in a web of lithium wires, which are about 1000 times smaller than a strand of hair. The electrode from battery 2 had a very smooth surface, meaning that lithium had grown into layers rather than wires. This proves that it is possible to control lithium electrodes, which is a very exciting new discovery. We hope that the new knowledge will help battery researchers develop new and improved batteries. It is our goal that this next general of batteries will store more energy so that they can power your phone or car for even longer before needing to be charged again.