The answer to this is largely a matter of personal opinion. There are not really any ethical principles connected to lying that are objectively correct. Instead, our attitudes towards the ethics of police deception in interrogation are determined more by our own beliefs than by any objectively provable ethical principles.
From one point of view, we can say that police act unethically when they lie during interrogations because they take away the suspect’s right to authentic self-determination. Most people would say that all individuals have the right to decide what is best for them and what courses of action they will take. When police lie (we can argue) they take that right away from a suspect. They give the suspect false information, thus making it impossible for him or her to make a truly informed decision about how to conduct themselves in the interrogation. The lies that the police tell cause the suspect to make their decision based on false information, thus making it impossible for them to truly decide in an independent way what they are going to do.
However, it is also possible to argue that no ethical principle has been violated when the police lie, so long as their lies are kept within legal bounds. The law tells police that they may not tell lies that would induce an innocent person to confess to a crime. They can only tell lies that would cause a guilty person to confess. We can argue that it is ethical to tell lies in pursuit of something that is valuable to society. When police tell lies, they are simply trying to protect our society as a whole. Because the lie is told in pursuit of a goal that is clearly appropriate and desirable, there is no ethical violation.
Neither of these points of view can be objectively proven to be true. You must decide for yourself which argument is more compelling.